About half of parents of children with ASD notice their child's unusual behaviors by age 18 months, and about four-fifths notice by age 24 months. According to an article, failure to meet any of the following milestones "is an absolute indication to proceed with further evaluations. Delay in referral for such testing may delay early diagnosis and treatment and affect the long-term outcome".

- No babbling by 12 months.

- No gesturing (pointing, waving, etc.) by 12 months.

- No single words by 16 months.

- No two-word (spontaneous, not just echolalic) phrases by 24 months.

- Any loss of any language or social skills, at any age.

The United States Preventive Services Task Force in 2016 found it was unclear if screening was beneficial or harmful among children in whom there is no concerns. The Japanese practice is to screen all children for ASD at 18 and 24 months, using autism-specific formal screening tests. In contrast, in the UK, children whose families or doctors recognize possible signs of autism are screened. It is not known which approach is more effective. Screening tools include the Modified Checklist for Autism in Toddlers (M-CHAT), the Early Screening of Autistic Traits Questionnaire, and the First Year Inventory; initial data on M-CHAT and its predecessor, the Checklist for Autism in Toddlers (CHAT), on children aged 18–30 months suggests that it is best used in a clinical setting and that it has low sensitivity (many false-negatives) but good specificity (few false-positives). It may be more accurate to precede these tests with a broadband screener that does not distinguish ASD from other developmental disorders. Screening tools designed for one culture's norms for behaviors like eye contact may be inappropriate for a different culture. Although genetic screening for autism is generally still impractical, it can be considered in some cases, such as children with neurological symptoms and dysmorphic features.

While infection with rubella during pregnancy causes fewer than 1% of cases of autism, vaccination against rubella can prevent many of those cases.

ASD can be detected as early as 18 months or even younger in some cases. A reliable diagnosis can usually be made by the age of two years. The diverse expressions of ASD symptoms pose diagnostic challenges to clinicians. Individuals with an ASD may present at various times of development (e.g., toddler, child, or adolescent), and symptom expression may vary over the course of development. Furthermore, clinicians must differentiate among pervasive developmental disorders, and may also consider similar conditions, including intellectual disability not associated with a pervasive developmental disorder, specific language disorders, ADHD, anxiety, and psychotic disorders.

Considering the unique challenges in diagnosing ASD, specific practice parameters for its assessment have been published by the American Academy of Neurology, the American Academy of Child and Adolescent Psychiatry, and a consensus panel with representation from various professional societies. The practice parameters outlined by these societies include an initial screening of children by general practitioners (i.e., "Level 1 screening") and for children who fail the initial screening, a comprehensive diagnostic assessment by experienced clinicians (i.e. "Level 2 evaluation"). Furthermore, it has been suggested that assessments of children with suspected ASD be evaluated within a developmental framework, include multiple informants (e.g., parents and teachers) from diverse contexts (e.g., home and school), and employ a multidisciplinary team of professionals (e.g., clinical psychologists, neuropsychologists, and psychiatrists).

After a child shows initial evidence of ASD tendencies, psychologists administer various psychological assessment tools to assess for ASD. Among these measurements, the Autism Diagnostic Interview-Revised (ADI-R) and the Autism Diagnostic Observation Schedule (ADOS) are considered the "gold standards" for assessing autistic children. The ADI-R is a semi-structured parent interview that probes for symptoms of autism by evaluating a child's current behavior and developmental history. The ADOS is a semistructured interactive evaluation of ASD symptoms that is used to measure social and communication abilities by eliciting several opportunities (or "presses") for spontaneous behaviors (e.g., eye contact) in standardized context. Various other questionnaires (e.g., The Childhood Autism Rating Scale, Autism Treatment Evaluation Checklist) and tests of cognitive functioning (e.g., The Peabody Picture Vocabulary Test) are typically included in an ASD assessment battery.

In the UK, there is some diagnostic use of the Diagnostic Interview for Social and Communication Disorders (DISCO) was which was developed for use at The Centre for Social and Communication Disorders, by Lorna Wing and Judith Gould, as both a clinical and a research instrument for use with children and adults of any age. The DISCO is designed to elicit a picture of the whole person through the story of their development and behaviour. In clinical work, the primary purpose is to facilitate understanding of the pattern over time of the specific skills and impairments that underlie the overt behaviour. If no information is available, the clinician has to obtain as much information as possible concerning the details of current skills and pattern of behaviour of the person. This type of dimensional approach to clinical description is useful for prescribing treatment.

PDD-NOS is an old diagnostic category. It is no longer included as an option for an Autism Spectrum Disorder and is not part of the DSM-5, but is included in the ICD-10.

The diagnosis of a pervasive developmental disorder not otherwise specified is given to individuals with difficulties in the areas of social interaction, communication, and/or stereotyped behavior patterns or interests, but who do not meet the full DSM-IV criteria for autism or another PDD. This does not necessarily mean that PDD-NOS is a milder disability than the other PDDs. It only means that individuals who receive this diagnosis do not meet the diagnostic criteria of the other PDDs, but that there is still a pervasive developmental disorder that affects the individual in the areas of communication, socialization and behavior.

As for the other pervasive developmental disorders, diagnosis of PDD-NOS requires the involvement of a team of specialists. The individual needs to undergo a full diagnostic evaluation, including a thorough medical, social, adaptive, motor skills and communication history. Other parts of an assessment can be behavioral rating scales, direct behavioral observations, psychological assessment, educational assessment, communication assessment, and occupational assessment.

Description of PDD-NOS merely as a "subthreshold" category without a more specific case definition poses methodological problems for research regarding the relatively heterogeneous group of people who receive this diagnosis. However, it appears that children with PDD-NOS show fewer intellectual deficits than autistic children, and that they may come to professional attention at a later age.

Parents of children with Asperger syndrome can typically trace differences in their children's development to as early as 30 months of age. Developmental screening during a routine check-up by a general practitioner or pediatrician may identify signs that warrant further investigation. The United States Preventive Services Task Force in 2016 found it was unclear if screening was beneficial or harmful among children in whom there are no concerns.

The diagnosis of AS is complicated by the use of several different screening instruments, including the Asperger Syndrome Diagnostic Scale (ASDS), Autism Spectrum Screening Questionnaire (ASSQ), Childhood Autism Spectrum Test (CAST) (previously called the Childhood Asperger Syndrome Test), Gilliam Asperger's disorder scale (GADS), Krug Asperger's Disorder Index (KADI), and the Autism-spectrum quotient (AQ; with versions for children, adolescents and adults). None have been shown to reliably differentiate between AS and other ASDs.

There is a division among doctors on the use of the term PDD. Many use the term PDD as a short way of saying PDD-NOS. Others use the general category because the term PDD actually refers to a category of disorders and is not a diagnostic label.

PDD is not itself a diagnosis, while PDD-NOS is a diagnosis. To further complicate the issue, PDD-NOS can also be referred to as "atypical personality development", "atypical PDD", or "atypical Autism".

Because of the "NOS", which means "not otherwise specified", it is hard to describe what PDD-NOS is, other than its being an autism spectrum disorder (ASD). Some people diagnosed with PDD-NOS are close to having Asperger syndrome, but do not quite fit. Others have near full-fledged autism, but without some of its symptoms. The psychology field is considering creating several subclasses within PDD-NOS.

The pervasive developmental disorders are:

- Pervasive developmental disorder not otherwise specified (PDD-NOS), which includes atypical autism, and is the most common (47% of diagnoses);

- Autism, the best-known;

- Asperger syndrome (9% of autism diagnoses);

- Rett syndrome; and

- Childhood disintegrative disorder (CDD).

The first three of these disorders are commonly called the autism spectrum disorders; the last two disorders are much rarer, and are sometimes placed in the autism spectrum and sometimes not.

In May 2013, the "Diagnostic and Statistical Manual-Fifth Edition" ("DSM-5") was released, updating the classification for pervasive developmental disorders. The grouping of disorders, including PDD-NOS, Autism, Asperger Syndrome, Rett Syndrome, and CDD, has been removed and replaced with the general term of Autism Spectrum Disorders. The American Psychiatric Association has concluded that using the general diagnosis of ASD supports more accurate diagnoses. The combination of these disorders was also fueled by the standpoint that Autism is characterized by common symptoms and should therefore bear a single diagnostic term. In order to distinguish between the different disorders, the DSM-5 employs severity levels. The severity levels take into account required support, restricted interests and repetitive behaviors, and deficits in social communication.

Studies suggest that persons with PDD-NOS belong to one of three very different subgroups:

- A high-functioning group (around 25 percent) whose symptoms largely overlap with that of Asperger syndrome, but who differ in terms of having a lag in language development and/or mild cognitive impairment. (The criteria for Asperger syndrome excludes a speech delay or a cognitive impairment.)

- A group (around 25 percent) whose symptoms more closely resemble those of autism spectrum disorder, but do not fully meet all its diagnostic signs and symptoms.

- The biggest group (around 50 percent) consists of those who meet all the diagnostic criteria for autism spectrum disorder, but whose stereotypical and repetitive behaviors are noticeably mild.

Standard diagnostic criteria require impairment in social interaction and repetitive and stereotyped patterns of behavior, activities and interests, without significant delay in language or cognitive development. Unlike the international standard, the DSM-IV-TR criteria also required significant impairment in day-to-day functioning; DSM-5 eliminated AS as a separate diagnosis in 2013, and folded it into the umbrella of autism spectrum disorders. Other sets of diagnostic criteria have been proposed by Szatmari "et al." and by Gillberg and Gillberg.

Diagnosis is most commonly made between the ages of four and eleven. A comprehensive assessment involves a multidisciplinary team that observes across multiple settings, and includes neurological and genetic assessment as well as tests for cognition, psychomotor function, verbal and nonverbal strengths and weaknesses, style of learning, and skills for independent living. The "gold standard" in diagnosing ASDs combines clinical judgment with the Autism Diagnostic Interview-Revised (ADI-R)—a semistructured parent interview—and the Autism Diagnostic Observation Schedule (ADOS)—a conversation and play-based interview with the child. Delayed or mistaken diagnosis can be traumatic for individuals and families; for example, misdiagnosis can lead to medications that worsen behavior.

Underdiagnosis and overdiagnosis may be problems. The cost and difficulty of screening and assessment can delay diagnosis. Conversely, the increasing popularity of drug treatment options and the expansion of benefits has motivated providers to overdiagnose ASD. There are indications AS has been diagnosed more frequently in recent years, partly as a residual diagnosis for children of normal intelligence who are not autistic but have social difficulties.

There are questions about the external validity of the AS diagnosis. That is, it is unclear whether there is a practical benefit in distinguishing AS from HFA and from PDD-NOS; the same child can receive different diagnoses depending on the screening tool. The debate about distinguishing AS from HFA is partly due to a tautological dilemma where disorders are defined based on severity of impairment, so that studies that appear to confirm differences based on severity are to be expected.

Developmental Verbal Dyspraxia can be diagnosed by a speech language pathologist (SLP) through specific exams that measure oral mechanisms of speech. The oral mechanisms exam involves tasks such as pursing lips, blowing, licking lips, elevating the tongue, and also involves an examination of the mouth. A complete exam also involves observation of the patient eating and talking. Tests such as the Kaufman Speech Praxis test, a more formal examination, are also used in diagnosis.

A differential diagnosis of DVD/CAS is often not possible for children under the age of 2 years old. Even when children are between 2–3 years, a clear diagnosis cannot always occur, because at this age, they may still be unable to focus on, or cooperate with, diagnostic testing.

Communication deviance (CD) occurs when a speaker fails to effectively communicate meaning to their listener with confusing speech patterns or illogical patterns. These disturbances can range from vague linguistic references, contradictory statements to more encompassing non-verbal problems at the level of turn-taking. The term was originally introduced by Wynne and Singer in 1963 to describe a communication style found among parents who had children with schizophrenia. A recent meta-analysis reported that communication deviance is highly prevalent in parents of patients diagnosed with schizophrenia and adoption studies have reported significant associations between CD in the parent and thought disorder in the offspring, however, the mechanisms by which CD impacts on the offspring's cognition are still unknown.

The research of psychiatrists and psychoanalysts Lyman Wynne and Theodore Lidz on communication deviance and roles (e.g., pseudo-mutuality, pseudo-hostility, schism and skew) in families of people with schizophrenia also became influential with "systems-communications"-oriented theorists and therapists.

Autism spectrum disorders tend to be highly comorbid with other disorders. Comorbidity may increase with age and may worsen the course of youth with ASDs and make intervention/treatment more difficult. Distinguishing between ASDs and other diagnoses can be challenging, because the traits of ASDs often overlap with symptoms of other disorders, and the characteristics of ASDs make traditional diagnostic procedures difficult.

The most common medical condition occurring in individuals with autism spectrum disorders is seizure disorder or epilepsy, which occurs in 11-39% of individuals with ASD. Tuberous sclerosis, a medical condition in which non-malignant tumors grow in the brain and on other vital organs, occurs in 1-4% of individuals with ASDs.

Intellectual disabilities are some of the most common comorbid disorders with ASDs. Recent estimates suggest that 40-69% of individuals with ASD have some degree of an intellectual disability, more likely to be severe for females. A number of genetic syndromes causing intellectual disability may also be comorbid with ASD, including fragile X syndrome, Down syndrome, Prader-Willi and Angelman syndromes, and Williams syndrome.

Learning disabilities are also highly comorbid in individuals with an ASD. Approximately 25-75% of individuals with an ASD also have some degree of a learning disability.

Various anxiety disorders tend to co-occur with autism spectrum disorders, with overall comorbidity rates of 7-84%. Rates of comorbid depression in individuals with an ASD range from 4–58%. The relationship between ASD and schizophrenia remains a controversial subject under continued investigation, and recent meta-analyses have examined genetic, environmental, infectious, and immune risk factors that may be shared between the two conditions.

Deficits in ASD are often linked to behavior problems, such as difficulties following directions, being cooperative, and doing things on other people's terms. Symptoms similar to those of attention deficit hyperactivity disorder (ADHD) can be part of an ASD diagnosis.

Sensory processing disorder is also comorbid with ASD, with comorbidity rates of 42–88%.

The first diagnosed case of ASD was published in 1943 by American psychiatrist Leo Kanner. There is a wide range of cases and severity to ASD so it is very hard to detect the first signs of ASD. A diagnosis of ASD can be made accurately before the child is 3 years old, but the diagnosis of ASD is not commonly confirmed until the child is somewhat older. The age of diagnosis can range from 9 months to 14 years, and the mean age is 4 years old in the USA. On average each case of ASD is tested at three different diagnostic centers before confirmed. Early diagnosis of the disorder can diminish familial stress, speed up referral to special educational programs and influence family planning.

Pragmatic language impairment (PLI), or social (pragmatic) communication disorder (SCD), is an impairment in understanding pragmatic aspects of language. This type of impairment was previously called semantic-pragmatic disorder (SPD). People with these impairments have special challenges with the semantic aspect of language (the meaning of what is being said) and the pragmatics of language (using language appropriately in social situations). It is assumed that those with autism have difficulty with "the meaning of what is being said" due to different ways of responding to social situations.

PLI is now a diagnosis in DSM-5, and is called social (pragmatic) communication disorder. Communication problems are also part of the autism spectrum disorders (ASD); however, the latter also show a restricted pattern of behavior, according to behavioral psychology. The diagnosis SCD can only be given if ASD has been ruled out.

For nonverbal grade school children and adolescents with autism, communication systems and interventions have been implemented to enhance language and communication outcomes. Speech-generated devices, such as iPads, use visual displays for children who lack verbal language, giving them the task of selecting icons indicating a request or need. For adolescents with nonverbal autism, interventions can condition them to learn more advanced operations on speech-generated devices that require more steps (i.e. turning on device, scrolling through pages), which would allow them to enhance their communicative abilities independently.

The Picture Exchange System (PECS) is an alternative form of spontaneous communication for children with autism in which an individual selects a picture indicating a request. PECS can be utilized in educational settings and at the child’s home. Longitudinal studies suggest PECS can have long-term positive outcomes for school-aged children with nonverbal autism, specifically their social-communicative skills, such as higher frequencies of joint attention and initiation, and duration of cooperative play, which are all important roles in improving language outcomes.

It has also been suggested that a significant stage in acquiring verbal language is learning how to identify and reproduce syllables of words. One study found that nonverbal and minimally verbal children with autism are capable of enhancing their oral production and vocalizing written words by isolating each syllable of a word one at a time. The process of breaking down a syllable at a time and having it visually displayed and audibly available to the child can prompt him or her to imitate and create nonrandom and meaningful utterances.

Most of these studies contain small sample sizes and were pilot studies, making additional research significant to assess whether these findings can be generalized to all age groups of the same population. Furthermore, most studies on nonverbal autism speech-generated device communication were based on more basic skills, such as naming pictures and making requests for stimuli, while studies in advanced communication (i.e. asking "how are you?") is limited.

According to the DSM-IV-TR, communication disorders are usually first diagnosed in childhood or adolescence though they are not limited as childhood disorders and may persist into adulthood. They may also occur with other disorders.

Diagnosis involves testing and evaluation during which it is determined if the scores/performance are "substantially below" developmental expectations and if they "significantly" interfere with academic achievement, social interactions and daily living. This assessment may also determine if the characteristic is deviant or delayed. Therefore, it may be possible for an individual to have communication challenges but not meet the criteria of being "substantially below" criteria of the DSM IV-TR.

It should also be noted that the DSM diagnoses do not comprise a complete list of all communication disorders, for example, auditory processing disorder is not classified under the DSM or ICD-10.

The following diagnoses are included in the communication disorders:

- Expressive language disorder – Characterized by difficulty expressing oneself beyond simple sentences and a limited vocabulary. An individual understands language better than their ability to use it; they may have a lot to say but have difficulties organizing and retrieving the words to get an idea across beyond what is expected for their developmental stage.

- Mixed receptive-expressive language disorder – problems comprehending the commands of others.

- Stuttering – a speech disorder characterized by a break in fluency, where sounds, syllables or words may be repeated or prolonged.

- Phonological disorder – a speech sound disorder characterized by problems in making patterns of sound errors, i.e. "dat" for "that".

- Communication disorder NOS (not otherwise specified) – the DSM-IV diagnosis in which disorders that do not meet the specific criteria for the disorder listed above may be classified.

In 1983, Rapin and Allen suggested the term "semantic pragmatic disorder" to describe the communicative behavior of children who presented traits such as pathological talkativeness, deficient access to vocabulary and discourse comprehension, atypical choice of terms and inappropriate conversational skills. They referred to a group of children who presented with mild autistic features and specific semantic pragmatic language problems. More recently, the term "pragmatic language impairment" (PLI) has been proposed.

Rapin and Allen's definition has been expanded and refined by therapists who include communication disorders that involve difficulty in understanding the meaning of words, grammar, syntax, prosody, eye gaze, body language, gestures, or social context. While autistic children exhibit pragmatic language impairment, this type of communication disorder can also be found in individuals with other types of disorders including auditory processing disorders, neuropathies, encephalopathies and certain genetic disorders.

Pathological Demand Avoidance is not recognised by the DSM-5 or ICD-10, the two main classification systems for mental disorder.

To be recognized a sufficient amount of consensus and clinical history needs to be present, and as a newly proposed condition PDA had not met the standard of evidence required at the time of recent revisions. In April 2014 the UK Minister of State for Care and Support Norman Lamb stated that the Department of Health, "In the course of the development of the National Institute for Health and Care Excellence (NICE) clinical guideline on the treatment of autism in children and young people (CG128), the developers looked at differential diagnoses for autism. In this, they did consider PDA, identifying it as a particular subgroup of autism that could also be described as oppositional defiant disorder (ODD). The guidance recommends that consideration should be given to differential diagnoses for autism (including ODD) and whether specific assessments are needed to help interpret the autism history and observations. However, due to the lack of evidence and the fact that the syndrome is not recognised within the DSM or ICD classifications, NICE was unable to develop specific recommendations on the assessment and treatment of PDA."

So the National Institute for Health and Care Excellence (which provides guidelines on best practice for UK clinicians) makes no mention of PDA in its guidelines for diagnosis of autism either in children or adults.

There is no cure for ASD and proper treatment depends on the case and what is most struggled with. Autism spectrum disorder is like many other disorders where when diagnosed early, can be better treated. Different types of therapy are helpful such as music therapy and physical therapy. Other treatments include auditory training, discrete trial training, facilitated communication, and sensory integration therapy.

It is estimated that 25 to 50% of children diagnosed with Autism Spectrum Disorder (ASD) never develop spoken language beyond a few words or utterances. Despite the growing field of research on ASD, there is not much information available pertaining to individuals with autism who never develop functional language; that, in fact, individuals with nonverbal autism are considered to be underrepresented in all of autism research. Because of the limited research on nonverbal autism, there are not many validated measurements appropriate for this population. For example, while they may be appropriate for younger children, they lack the validity for grade-school aged children and adolescents and have continued to be a roadblock for nonverbal autism research. Often in autism research, individuals with nonverbal autism are sub-grouped with LFA, categorized by learning at most one word or having minimal verbal language.

Most of the existing body of research in nonverbal autism focuses on early interventions that predict successful language outcomes. Research suggests that most spoken language is inherited before the age of five, and the likelihood of acquiring functional language in the future past this age is minimal, that early language development is crucial to educational achievement, employment, independence during adulthood, and social relationships.

Mental retardation is coded on Axis II of the DSM-IV-TR. The diagnostic criteria necessary in order to diagnose intellectual disability consists of:

There are varying degrees of intellectual disability, which are identified by an IQ test.

Mental retardation, Severity Unspecified: This unspecified diagnosis is given when there is a strong assumption that the child is mentally retarded, but cannot be tested because the individual is too impaired, not willing to take the IQ test or is an infant.

Disorders and tendencies included and excluded under the category of communication disorders may vary by source. For example, the definitions offered by the American Speech–Language–Hearing Association differ from that of the Diagnostic Statistical Manual 4th edition (DSM-IV).

Gleanson (2001) defines a communication disorder as a speech and language disorder which refers to problems in communication and in related areas such as oral motor function. The delays and disorders can range from simple sound substitution to the inability to understand or use their native language.

In general, communications disorders commonly refer to problems in speech (comprehension and/or expression) that significantly interfere with an individual’s achievement and/or quality of life. Knowing the operational definition of the agency performing an assessment or giving a diagnosis may help.

Persons who speak more than one language or are considered to have an accent in their location of residence do not have speech disorders if they are speaking in a manner consistent with their home environment or a blending of their home and foreign environment.

Intellectual disability in children can be caused by genetic or environmental factors. The individual could have a natural brain malformation or pre or postnatal damage done to the brain caused by drowning or a traumatic brain injury, for example. Nearly 30 to 50% of individuals with intellectual disability will never know the cause of their diagnosis even after thorough investigation.

Prenatal causes of intellectual disability include:

- Congenital infections such as cytomegalovirus, toxoplasmosis, herpes, syphilis, rubella and human immunodeficiency virus

- Prolonged maternal fever in the first trimester

- Exposure to anticonvulsants or alcohol

- Untreated maternal phenylketonuria (PKU)

- Complications of prematurity, especially in extremely low-birth-weight infants

- Postnatal exposure to lead

Single-gene disorders that result in intellectual disability include:

- Fragile X syndrome

- Neurofibromatosis

- Tuberous sclerosis

- Noonan's syndrome

- Cornelia de Lange's syndrome

These single-gene disorders are usually associated with atypical physical characteristics.

About 1/4 of individuals with intellectual disability have a detectable chromosomal abnormality. Others may have small amounts of deletion or duplication of chromosomes, which may go unnoticed and therefore, undetermined.

Pathological demand avoidance (PDA) is a proposed subtype of autism characterized by an avoidance of demand-framed requests by an individual. It was proposed in 1980 by the UK child psychologist Elizabeth Newson. The Elizabeth Newson Centre in Nottingham, England carries out assessments for the NHS, local authorities and private patients around autism spectrum disorder, which include, but are not exclusively PDA.

PDA behaviours are consistent with autism, but have differences from other autism subtypes diagnoses. It is not recognised by either the DSM-5 or the .

Specific developmental disorders are disorders in which development is delayed in one specific area or areas, and in which basically all other areas of development are not affected. Specific developmental disorders are as opposed to pervasive developmental disorders that are characterized by delays in the development of multiple basic functions including socialization and communication.