As a pervasive developmental disorder, Asperger syndrome is distinguished by a pattern of symptoms rather than a single symptom. It is characterized by qualitative impairment in social interaction, by stereotyped and restricted patterns of behavior, activities and interests, and by no clinically significant delay in cognitive development or general delay in language. Intense preoccupation with a narrow subject, one-sided verbosity, restricted prosody, and physical clumsiness are typical of the condition, but are not required for diagnosis. Suicidal behavior appears to occur at rates similar to those without ASD.

Autism is a highly variable neurodevelopmental disorder that first appears during infancy or childhood, and generally follows a steady course without remission. People with autism may be severely impaired in some respects but normal, or even superior, in others. Overt symptoms gradually begin after the age of six months, become established by age two or three years, and tend to continue through adulthood, although often in more muted form. It is distinguished not by a single symptom, but by a characteristic triad of symptoms: impairments in social interaction; impairments in communication; and restricted interests and repetitive behavior. Other aspects, such as atypical eating, are also common but are not essential for diagnosis. Autism's individual symptoms occur in the general population and appear not to associate highly, without a sharp line separating pathologically severe from common traits.

Autistic individuals may have symptoms that are independent of the diagnosis, but that can affect the individual or the family.

An estimated 0.5% to 10% of individuals with ASD show unusual abilities, ranging from splinter skills such as the memorization of trivia to the extraordinarily rare talents of prodigious autistic savants. Many individuals with ASD show superior skills in perception and attention, relative to the general population. Sensory abnormalities are found in over 90% of those with autism, and are considered core features by some, although there is no good evidence that sensory symptoms differentiate autism from other developmental disorders. Differences are greater for under-responsivity (for example, walking into things) than for over-responsivity (for example, distress from loud noises) or for sensation seeking (for example, rhythmic movements). An estimated 60%–80% of autistic people have motor signs that include poor muscle tone, poor motor planning, and toe walking; deficits in motor coordination are pervasive across ASD and are greater in autism proper.

Unusual eating behavior occurs in about three-quarters of children with ASD, to the extent that it was formerly a diagnostic indicator. Selectivity is the most common problem, although eating rituals and food refusal also occur; this does not appear to result in malnutrition. Although some children with autism also have gastrointestinal symptoms, there is a lack of published rigorous data to support the theory that children with autism have more or different gastrointestinal symptoms than usual; studies report conflicting results, and the relationship between gastrointestinal problems and ASD is unclear.

Parents of children with ASD have higher levels of stress. Siblings of children with ASD report greater admiration of and less conflict with the affected sibling than siblings of unaffected children and were similar to siblings of children with Down syndrome in these aspects of the sibling relationship. However, they reported lower levels of closeness and intimacy than siblings of children with Down syndrome; siblings of individuals with ASD have greater risk of negative well-being and poorer sibling relationships as adults.

The extent of the overlap between AS and high-functioning autism (HFA—autism unaccompanied by intellectual disability) is unclear. The ASD classification is to some extent an artifact of how autism was discovered, and may not reflect the true nature of the spectrum; methodological problems have beset Asperger syndrome as a valid diagnosis from the outset. In the fifth edition of the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-5), published in May 2013, AS, as a separate diagnosis, was eliminated and folded into autism spectrum disorder. Like the diagnosis of Asperger syndrome, the change was controversial and AS was not removed from the WHO's ICD-10.

The World Health Organization (WHO) defines Asperger syndrome (AS) as one of the autism spectrum disorders (ASD) or pervasive developmental disorders (PDD), which are a spectrum of psychological conditions that are characterized by abnormalities of social interaction and communication that pervade the individual's functioning, and by restricted and repetitive interests and behavior. Like other psychological development disorders, ASD begins in infancy or childhood, has a steady course without remission or relapse, and has impairments that result from maturation-related changes in various systems of the brain. ASD, in turn, is a subset of the broader autism phenotype, which describes individuals who may not have ASD but do have autistic-like traits, such as social deficits. Of the other four ASD forms, autism is the most similar to AS in signs and likely causes, but its diagnosis requires impaired communication and allows delay in cognitive development; Rett syndrome and childhood disintegrative disorder share several signs with autism but may have unrelated causes; and pervasive developmental disorder not otherwise specified (PDD-NOS) is diagnosed when the criteria for a more specific disorder are unmet.

Approximately 25–30% of children with autism spectrum disorders stop speaking after beginning to say words, often before the age of two. According to Ami Klin, "Most examples of autistic regression... are based upon a child's loss of a handful of words... it's possible that these children were only echoing sounds they heard from their parents" Some children lose social development instead of language; some lose both. After the regression, the child follows the standard pattern of autistic neurological development. The term refers to the appearance that neurological development has reversed; it is actually only the affected developmental skills, rather than the neurology as a whole, that regresses. It is more usual for autistic neurological development to not include such aberrations, with age-appropriate autistic symptoms being clear from birth.

Skill loss may be quite rapid, or may be slow and preceded by a lengthy period of no skill progression; the loss may be accompanied by reduced social play or increased irritability. The temporarily acquired skills typically amount to a few words of spoken language, and may include some rudimentary social perception. There are several intermediate types of development, which do not neatly fit into either the traditional early onset or the regressive categories, including mixtures of early deficits, failures to progress, subtle diminishments, and obvious losses. If regression is defined strictly to require loss of language, it is less common; if defined more broadly, to include cases where language is preserved but social interaction is diminished, it is more common.

Under the DSM-5, autism is characterized by persistent deficits in social communication and interaction across multiple contexts, as well as restricted, repetitive patterns of behavior, interests, or activities. These deficits are present in early childhood, and lead to clinically significant functional impairment. There is also a unique form of autism called autistic savantism, where a child can display outstanding skills in music, art, and numbers with no practice. Because of its relevance to different populations, self-injurious behaviors (SIB) are not considered a core characteristic of the ASD population however approximately 50% of those with ASD take part in some type of SIB (head-banging, self-biting) and are more at risk than other groups with developmental disabilities.

Other characteristics of ASD include Restricted and Repetitive Behaviors (RRBs) which include a large range of specific gestures and acts, it can even include certain behavioral traits as defined in the Diagnostic and Statistic Manual for Mental Disorders.

Asperger syndrome was distinguished from autism in the DSM-IV by the lack of delay or deviance in early language development. Additionally, individuals diagnosed with Asperger syndrome did not have significant cognitive delays. PDD-NOS was considered "subthreshold autism" and "atypical autism" because it was often characterized by milder symptoms of autism or symptoms in only one domain (such as social difficulties).

The DSM-5 eliminated the four separate diagnoses: Asperger Syndrome, Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS), Childhood Degenerative Disorder, and Autistic Disorder and combined them under the diagnosis of Autism Spectrum Disorder.

CDD is a rare condition, with only 1.7 cases per 100,000.

A child affected with childhood disintegrative disorder shows normal development and he/she acquires "normal development of age-appropriate verbal and nonverbal communication, social relationships, motor, play and self-care skills" comparable to other children of the same age. However, between the ages of 2 and 10, skills acquired are lost almost completely in at least two of the following six functional areas:

- Expressive language skills (being able to produce speech and communicate a message)

- Receptive language skills (comprehension of language - listening and understanding what is communicated)

- Social skills and self care skills

- Control over bowel and bladder

- Play skills

- Motor skills

Lack of normal function or impairment also occurs in at least two of the following three areas:

- Social interaction

- Communication

- Repetitive behavior and interest patterns

In her book, "Thinking in Pictures", Temple Grandin argues that compared to "Kanner's classic autism" and to Asperger syndrome, CDD is characterized with more severe sensory processing disorder but less severe cognitive problems. She also argues that compared to most autistic people, persons with CDD have more severe speech pathology and they usually do not respond well to stimulants.

Disorders considered in origin include:

1. Intellectual disability (ID) or intellectual and developmental disability (IDD), previously called mental retardation

2. Autism spectrum disorders, such as Asperger's syndrome or Kanner syndrome

3. Motor disorders including developmental coordination disorder and stereotypic movement disorder Tic disorders including Tourette's syndrome

4. Traumatic brain injury (including congenital injuries such as those that cause cerebral palsy)

5. Communication, speech and language disorders

6. genetic disorders, such as fragile-X syndrome, Down syndrome, attention deficit hyperactivity disorder, schizophrenia, schizotypal disorder, hypogonadotropic hypogonadal syndromes

7. disorders due to neurotoxicants like fetal alcohol spectrum disorder, Minamata disease caused by mercury, behavioral disorders including conduct disorder etc caused by other heavy metals, such as lead, chromium, platinum etc, hydrocarbons like dioxin, PBDEs and PCBs, medications and illegal drugs, like cocaine and others.

In the United States, a revision to autism spectrum disorder (ASD) was presented in the "Diagnostic and Statistical Manual of Mental Disorders" version 5 (DSM-5), released May 2013. The new diagnosis encompasses previous diagnoses of autistic disorder, Asperger's disorder, childhood disintegrative disorder, and PDD-NOS. Compared with the DSM-IV diagnosis of autistic disorder, the DSM-5 diagnosis of ASD no longer includes communication as a separate criterion, and has merged social interaction and communication into one category. Slightly different diagnostic definitions are used in other countries. For example, the ICD-10 is the most commonly-used diagnostic manual in the UK and European Union.

Rather than categorizing these diagnoses, the DSM-5 has adopted a dimensional approach to diagnosing disorders that fall underneath the autism spectrum umbrella. Some have proposed that individuals on the autism spectrum may be better represented as a single diagnostic category. Within this category, the DSM-5 has proposed a framework of differentiating each individual by dimensions of severity, as well as associated features (i.e., known genetic disorders, and intellectual disability).

Another change to the DSM includes collapsing social and communication deficits into one domain. Thus, an individual with an ASD diagnosis will be described in terms of severity of social communication symptoms, severity of fixated or restricted behaviors or interests, and associated features. The restricting of onset age has also been loosened from 3 years of age to "early developmental period", with a note that symptoms may manifest later when social demands exceed capabilities.

Autism forms the core of the autism spectrum disorders. Asperger syndrome is closest to autism in signs and likely causes; unlike autism, people with Asperger syndrome usually have no significant delay in language development, according to the older DSM-4 criteria. PDD-NOS is diagnosed when the criteria are not met for a more specific disorder. Some sources also include Rett syndrome and childhood disintegrative disorder, which share several signs with autism but may have unrelated causes; other sources differentiate them from ASD, but group all of the above conditions into the pervasive developmental disorders.

Autism, Asperger syndrome, and PDD-NOS are sometimes called the "autistic disorders" instead of ASD, whereas autism itself is often called "autistic disorder", "childhood autism", or "infantile autism". Although the older term "pervasive developmental disorder" and the newer term "autism spectrum disorder" largely or entirely overlap, the earlier was intended to describe a specific set of diagnostic labels, whereas the latter refers to a postulated spectrum disorder linking various conditions. ASD is a subset of the broader autism phenotype (BAP), which describes individuals who may not have ASD but do have autistic-like traits, such as avoiding eye contact.

The term "multisystem developmental disorder" has also been used to describe various developmental disorders. These include:

- Alagille syndrome, an autosomal dominant disorder with a wide range of features and manifestations. Its five most significant features are chronic cholestasis, a condition where bile cannot flow from the liver to the duodenum, occurring in 95% of cases; heart abnormalities (over 90%); butterfly vertebrae; posterior embryotoxon and a distinctive face (prominent forehead, deep-set eyes, and a pointed chin).

- Rubinstein-Taybi syndrome, a mental retardation syndrome characterized by broad thumbs, facial abnormalities, and big toes alongside mental retardation.

- Williams syndrome, a neurodevelopmental disorder characterized by a unique profile of strengths and deficits; most with the condition have mild mental retardation but have grammatical and lexical abilities above what would be expected from their IQs. They are hypersocial and empathetic, but social isolation is commonly experienced.

- Proteus syndrome, a congenital disorder causing disproportionate growth of skin, bone, and other tissues.

- Asphyxiating thoracic dysplasia, a autosomal recessive skeletal disorder with an estimated prevalence of between 1 in 100,000 and 1 in 130,000 live births.

Children with PDD vary widely in abilities, intelligence, and behaviors. Some children do not speak at all, others speak in limited phrases or conversations, and some have relatively normal language development. Repetitive play skills and limited social skills are generally evident as well. Unusual responses to sensory information – loud noises, lights – they also are common.

Regressive autism occurs when a child appears to develop typically but then starts to lose speech and social skills, typically between the ages of 15 and 30 months, and is subsequently diagnosed with autism. Other terms used to describe regression in children with autism are autism with regression, autistic regression, setback-type autism, and acquired autistic syndrome. There is no standard definition for regression, and the prevalence of regression varies depending on the definition used. Some children show a mixture of features, with some early delays and some later losses; and there is evidence of a continuous spectrum of behaviors, rather than a black-and-white distinction, between autism with and without regression. According to the definitions in the DSM-5 the term "regressive autism" can refer to any type of autism spectrum disorder that involves regression, including Childhood Disintegrative Disorder.

The childhood disintegrative disorder (CDD), also known as Heller's syndrome and disintegrative psychosis, is a rare condition characterized by late onset of developmental delays—or stunning reversals—in language, social function, and motor skills. Researchers have not been successful in finding a cause for the disorder. CDD has some similarity to autism, and is sometimes considered a low-functioning form of it. In May 2013, the term CDD, along with other types of autism, was fused into a single diagnostic term called "autism spectrum disorder" under the new DSM-5 manual. Therefore, CDD is now also called "regressive autism", being that this term can now refer to any type of autism spectrum disorder that involves regression, including CDD.

CDD was originally described by Austrian educator Theodor Heller (1869–1938) in 1908, 35 years before Leo Kanner and Hans Asperger described autism. Heller had previously used the name "dementia infantilis" for the syndrome.

An apparent period of fairly normal development is often noted before a regression in skills or a series of regressions in skills. The age at which this regression can occur varies, but typically after 3 years of normal development. The regression can be so dramatic that the child may be aware of it, and may in its beginning even ask, vocally, what is happening to them. Some children describe or appear to be reacting to hallucinations, but the most obvious symptom is that skills apparently attained are lost.

Many children are already somewhat delayed when the disorder becomes apparent, but these delays are not always obvious in young children. This has been described by many writers as a devastating condition, affecting both the family and the individual's future. As is the case with all pervasive developmental disorder categories, there is considerable controversy about the right treatment for CDD.

Symptoms of PDD may include behavioral and communication problems such as:

- Difficulty using and understanding language.

- Difficulty relating to people, objects, and events; for example, lack of eye contact, pointing behavior, and lack of facial responses.

- Unusual play with toys and other objects.

- Difficulty with changes in routine or familiar surroundings.

- Repetitive body movements or behavior patterns, such as hand flapping, hair twirling, foot tapping, or more complex movements.

- Inability to cuddle or be comforted.

- Difficulty regulating behaviors and emotions, which may result in temper tantrums, anxiety, and aggression.

- Emotional breakdowns.

Savant syndrome is a condition in which a person demonstrates one or more profound and prodigious capacities or abilities far in excess of what would be considered normal, yet often also has significant deficits in other areas of brain processing.

People with savant syndrome may have neurodevelopmental disorders, notably autism spectrum disorders (in which case they are often referred to as autistic savants), or brain injuries. The most dramatic examples of savant syndrome occur in individuals who score very low on IQ tests, while demonstrating exceptional skills or brilliance in specific areas, such as rapid calculation (hypercalculia), art, memory, or musical ability. Although termed a syndrome, it is not recognized as a mental disorder nor as part of a mental disorder in medical manuals such as the ICD-10 or the DSM-5.

Another form of savant syndrome is acquired savant syndrome, in which a person acquires prodigious capabilities or skills following dementia, a head injury or concussion, epilepsy, or other brain disturbances. This syndrome is more rare, with a study by Darold Treffert in 2010 showing that in a registry of 319 known savants, only 32 had acquired savant syndrome.

Hemimegalencephaly is an extremely rare form of macrocephaly and is characterized by uneven development of brain hemispheres (one-half of brain is larger than other). The syndrome can be presented by itself or in association with phakomatosis or hemigigantism. Additionally, hemimegalencephaly will frequently cause severe epilepsy, focal neuro-logical deficits, macrocrania, and mild to severe mental retardation.

Megalencephaly-capillary (MCAP) is one of the two major syndromes of megalencephaly. Typically, MCAP and MPPH can be distinguished by somatic features. MCAP includes many characteristics that are observed at birth including: cutaneous vascular malformations, especially capillary malformations of the face and cutis marmorata, polydactyly, connective tissue dysplasia, and focal or segmental body overgrowth. Furthermore, MCAP can occasionally be linked with asymmetric brain overgrowth (hemimegalencephaly) as well as segmental overgrowth of the body (hemihypertrophy).

Neurodevelopmental disorders are impairments of the growth and development of the brain or central nervous system. A narrower use of the term refers to a disorder of brain function that affects emotion, learning ability, self-control and memory and that unfolds as an individual develops and grows.

The term is sometimes erroneously used as an exclusive synonym for autism spectrum disorders.

Savant skills are usually found in one or more of five major areas: art, memory, arithmetic, musical abilities, and spatial skills.

The most common kind of savants are calendrical savants, "human calendars" who can calculate the day of the week for any given date with speed and accuracy, or recall personal memories from any given date. Advanced memory is the key "superpower" in savant abilities.

Approximately half of savants are autistic; the other half often have some form of central nervous system injury or disease.

It is estimated that 10% of those with autism have some form of savant abilities.

It is common for individuals with PDD-NOS to have more intact social skills and a lower level of intellectual deficit than individuals with other PDDs. Characteristics of many individuals with PDD-NOS are:

- Communication difficulties (e.g., using and understanding language)

- Difficulty with social behavior

- Difficulty with changes in routines or environments

- Uneven skill development (strengths in some areas and delays in others)

- Unusual play with toys and other objects

- Repetitive body movements or behavior patterns

- Preoccupation with fantasy, such as imaginary friends in childhood

High-functioning autism is characterized by features very similar to those of Asperger syndrome. The defining characteristic most widely recognized by psychologists is a significant delay in the development of early speech and language skills, before the age of three years. The diagnostic criteria of Asperger syndrome exclude a general language delay.

Further differences in features between people with high-functioning autism and those with Asperger syndrome, include the following:

- People with HFA have a lower verbal reasoning ability

- Better visual/spatial skills (higher performance IQ) than people with Asperger syndrome

- Less deviating locomotion than people with Asperger syndrome

- People with HFA more often have problems functioning independently

- Curiosity and interest for many different things, in contrast to people with Asperger syndrome

- People with Asperger syndrome are better at empathizing with another

- The male to female ratio of 4:1 for HFA is much smaller than that of Asperger syndrome

Individuals with autism spectrum disorders, including high-functioning autism, risk developing symptoms of anxiety. While anxiety is one of the most commonly occurring mental health symptoms, children and adolescents with high functioning autism are at an even greater risk of developing symptoms.

There are other comorbidities, the presence of one or more disorders in addition to the primary disorder, associated with high-functioning autism. Some of these include depression, bipolar disorder, and obsessive compulsive disorder (OCD). In particular the link between HFA and OCD, has been studied; both have abnormalities associated with serotonin.

Observable comorbidities associated with HFA include ADHD, Tourette syndrome, and possibly criminal behavior. While the association between HFA and criminal behavior is not completely characterized, several studies have shown that the features associated with HFA may increase the probability of engaging in criminal behavior. While there is still a great deal of research that needs to be done in this area, recent studies on the correlation between HFA and criminal actions suggest that there is a need to understand the attributes of HFA that may lead to violent behavior. There have been several case studies that link the lack of empathy and social naïveté associated with HFA to criminal actions.

HFA does not cause nor include intellectual disabilities. This characteristic distinguishes HFA from the rest of the autism spectrum; between 40 and 55% of individuals with autism also have an intellectual disability.

Multisystem developmental disorder (MSDD) is a term used by Stanley Greenspan to describe children under age 3 who exhibit signs of impaired communication as in autism, but with strong emotional attachments atypical of autism. It is described in the DC:0-3R manual as an optional diagnosis for children under two years of age.

Attention deficit hyperactivity disorder (ADHD) is found in the majority of males with FXS and 30% of females, making it the most common psychiatric diagnosis in those with FXS. Hyperactivity and disruptive behavior peak in the preschool years and then gradually decline with age, although inattentive symptoms are generally lifelong.

Aside from the characteristic social phobia features, a range of other anxiety symptoms are very commonly associated with FXS, with symptoms typically spanning a number of psychiatric diagnoses but not fulfilling any of the criteria in full. Behaviors such as hand flapping and biting, as well as aggression, can be an expression of anxiety. Although only a minority will meet the criteria for obsessive–compulsive disorder (OCD), a significant majority will feature obsessive-type symptoms. However, as individuals with FXS generally find these behaviors pleasurable, unlike individuals with OCD, they are more frequently referred to as stereotypic behaviors.

Mood symptoms in individuals with FXS rarely meet diagnostic criteria for a major mood disorder as they are typically not of sustained duration. Instead, these are usually transient and related to stressors, and may involve labile (fluctuating) mood, irritability, self-injury and aggression.

Individuals with fragile X-associated tremor/ataxia syndrome (FXTAS) are likely to experience combinations of dementia, mood, and anxiety disorders. Males with the "FMR1" premutation and clinical evidence of FXTAS were found to have increased occurrence of somatization, obsessive–compulsive disorder, interpersonal sensitivity, depression, phobic anxiety, and psychoticism.

Children with fragile X have very short attention spans, are hyperactive, and show hypersensitivity to visual, auditory, tactile, and olfactory stimuli. These children have difficulty in large crowds due to the loud noises and this can lead to tantrums due to hyperarousal. Children with FXS pull away from light touch and can find textures of materials to be irritating. Transitions from one location to another can be difficult for children with FXS. Behavioral therapy can be used to decrease the child’s sensitivity in some cases.

Perseveration is a common communicative and behavioral characteristic in FXS. Children with FXS may repeat a certain ordinary activity over and over. In speech, the trend is not only in repeating the same phrase but also talking about the same subject continually. Cluttered speech and self-talk are commonly seen. Self-talk includes talking with oneself using different tones and pitches.

Late talker is a term used for exceptionally bright people who experience a delay in the development of speech. Commonalities include usually being boys, delayed speech development, highly educated parents, musically gifted families, puzzle-solving abilities, and lagging social development. Many high-achieving late talkers were notoriously strong willed and noncompliant as children. Late talkers can often be misdiagnosed early on as having severe ("low-functioning") autism spectrum disorder (a category known simply as "autism", prior to the DSM-5), and careful professional evaluation is necessary for differential diagnosis, according to Darold Treffert and other experts. One major difference between late talkers and low-functioning autistic children is that for late talkers, communication skills automatically reach a normal level and the child requires no further special treatment with regards to speech. Outlook for late talkers with or without intervention is generally favorable. However, late language emergence can also be an early or secondary sign of high-functioning autism spectrum disorder / Asperger syndrome, or other developmental disorders, such as attention deficit hyperactivity disorder, intellectual disability, learning disability, social communication disorder, or specific language impairment.

Einstein syndrome, a term coined by the economist Thomas Sowell, is also sometimes used to describe late talkers. The term is named after Albert Einstein (often said to have been a late talker, though with questionable evidence), whom Sowell used as the primary example of a late talker in his work. Sowell also included Edward Teller, Srinivasa Ramanujan, the mathematician Julia Robinson, Richard Feynman, and the pianists Clara Schumann and Arthur Rubinstein to be in the late talkers group. As a toddler, the scientist John Clive Ward showed similar behavioral traits to those described by Sowell, according to a brief sketch of his biography.

Sowell claimed late talkers are often inaccurately categorized as having an autism spectrum disorder (ASD), and that a small subset of late talkers are highly intelligent children with common characteristics concentrated in music, memory, math or the sciences. However, as reported by Simon Baron-Cohen, such characteristics are often found in high-functioning autism / Asperger syndrome.