Language delays are the most frequent developmental delays, and can occur for many reasons. A delay can be due to being a “late bloomer,” or a more serious problem. The most common causes of speech delay include

- Hearing loss

- Slow development

- Intellectual Disability

Such delays can occur in conjunction with a lack of mirroring of facial responses, unresponsiveness or unawareness of certain noises, a lack of interest in playing with other children or toys, or no pain response to stimuli.

Other causes include:

- Psychosocial deprivation - The child doesn't spend enough time talking with adults. Research on early brain development shows that babies and toddlers have a critical need for direct interactions with parents and other significant care givers for healthy brain growth and the development of appropriate social, emotional, and cognitive skills.

- Television viewing is associated with delayed language development. Children who watched television alone were 8.47 times more likely to have language delay when compared to children who interacted with their caregivers during television viewing. As recommended by the American Academy of Pediatrics (AAP), children under the age of 2 should watch no television at all, and after age 2 watch no more than one to two hours of quality programming a day. Therefore, exposing such young children to television programs should be discouraged. Parents should engage children in more conversational activities to avoid television-related delays to their children language development, which could impair their intellectual performance.

- Stress during pregnancy is associated with language delay.

- Being a twin

- Attention deficit hyperactivity disorder

- Autism (a developmental disorder) - There is strong evidence that autism is commonly associated with language delay. Asperger syndrome, which is on the autistic spectrum, however, is not associated with language delay.

- Selective mutism (the child just doesn't want to talk)

- Cerebral palsy (a movement disorder caused by brain damage)

- Genetic abnormalities - In 2005, researchers found a connection between expressive language delay and a genetic abnormality: a duplicate set of the same genes that are missing in sufferers of Williams-Beuren syndrome. Also so called XYY syndrome can often cause speech delay.

- Correlation with male sex, previous family history, and maternal education has been demonstrated.

Specific language impairment (SLI) is diagnosed when a child has delayed or disordered language development for no apparent reason. Usually the first indication of SLI is that the child is later than usual in starting to speak and subsequently is delayed in putting words together to form sentences. Spoken language may be immature. In many children with SLI, understanding of language, or "receptive" language, is also impaired, though this may not be obvious unless the child is given a formal assessment.

Although difficulties with use and understanding of complex sentences are a common feature of SLI, the diagnostic criteria encompass a wide range of problems, and for some children other aspects of language are problematic (see below). In general, the term SLI is reserved for children whose language difficulties persist into school age, and so it would not be applied to toddlers who are late to start talking, most of whom catch up with their peer group after a late start.

In 2006, the U.S. Department of Education indicated that more than 1.4 million students were served in the public schools' special education programs under the speech or language impairment category of IDEA 2004. This estimate does not include children who have speech/language problems secondary to other conditions such as deafness; this means that if all cases of speech or language impairments were included in the estimates, this category of impairment would be the largest. Another source has estimated that communication disorders—a larger category, which also includes hearing disorders—affect one of every 10 people in the United States.

ASHA has cited that 24.1% of children in school in the fall of 2003 received services for speech or language disorders—this amounts to a total of 1,460,583 children between 3 –21 years of age. Again, this estimate does not include children who have speech/language problems secondary to other conditions. Additional ASHA prevalence figures have suggested the following:

- Stuttering affects approximately 4% to 5% of children between the ages of 2 and 4.

- ASHA has indicated that in 2006:

- Almost 69% of SLPs served individuals with fluency problems.

- Almost 29% of SLPs served individuals with voice or resonance disorders.

- Approximately 61% of speech-language pathologists in schools indicated that they served individuals with SLI

- Almost 91% of SLPs in schools indicated that they servedindividuals with phonological/articulation disorder

- Estimates for language difficulty in preschool children range from 2% to 19%.

- Specific Language Impairment (SLI) is extremely common in children, and affects about 7% of the childhood population.

Specific language impairment (SLI) is diagnosed when a child's language does not develop normally and the difficulties cannot be accounted for by generally slow development, physical abnormality of the speech apparatus, autism spectrum disorder, apraxia, acquired brain damage or hearing loss. Twin studies have shown that it is under genetic

influence. Although language impairment can result from a single-gene mutation, this is unusual. More commonly SLI results from the combined influence of multiple genetic variants, each of which is found in the general population, as well as environmental influences.

Studies have failed to find clear evidence that language delay can be prevented by training or educating health care professionals in the subject. Overall, some of the reviews show positive results regarding interventions in language delay, but are not curative. (Commentary - Early Identification of Language Delays, 2005)

A speech sound disorder is a speech disorder in which some speech sounds (called phonemes) in a child's (or, sometimes, an adult's) language are either not produced, not produced correctly, or are not used correctly. The term protracted phonological development is sometimes preferred when describing children's speech to emphasize the continuing development while acknowledging the delay.

Most speech sound disorders occur without a known cause. A child may not learn how to produce sounds correctly or may not learn the rules of speech sounds on his or her own. These children may have a problem with speech development, which does not always mean that they will simply outgrow it by themselves. Many children do develop speech sounds over time but those who do not often need the services of a Speech-Language Pathologist to learn correct speech sounds.

Some speech sound errors can result from other syndromes or disorders such as:

- developmental disorders (e.g. autism)

- genetic disorders (e.g. Down syndrome)

- hearing loss, including temporary hearing loss, such as from ear infections

- cleft palate or other physical anomalies of the mouth

- illness

- neurological disorders (e.g. cerebral palsy)

Speech delay, also known as alalia, refers to a delay in the development or use of the mechanisms that produce speech. Speech, as distinct from language, refers to the actual process of making sounds, using such organs and structures as the lungs, vocal cords, mouth, tongue, teeth, etc. Language delay refers to a delay in the development or use of the knowledge of language.

Because language and speech are two independent stages, they may be individually delayed. For example, a child may be delayed in speech (i.e., unable to produce intelligible speech sounds), but not delayed in language. In this case, the child would be attempting to produce an age appropriate amount of language, but that language would be difficult or impossible to understand. Conversely, since a child with a language delay typically has not yet had the opportunity to produce speech sounds, it is likely to have a delay in speech as well.

At times, speech delay and impairment is caused by a physical disruption in the mouth such as a deformed frenulum, lips, or palate. If the motion or ability to form words and appropriate sounds is disrupted, the child may be slow to pick up words and lack the ability to shape their mouth and tongue in the formation of words.

Other more serious concerns are those that can be caused by oral-motor issues. Oral-motor dysfunction refers to a lack or delay in the area of the brain that speech is formed and created and communicated to the mouth and tongue. While speech may be the only concern, this disorder can be highlighted with feeding issues as well.

Children that are having speech delay disorders could have the following characteristics (Shriberg 1982):

- Speech mechanism in which speech is associated with hearing, motor speech and craniofacial malfunction

- Cognitive-linguistic aspects in which the impairment is associated with the child's intellectual, receptive, expressive and linguistic ability.

- Psychosocial issues in which the impairment is associated with caregiver, school environment, and the child's self behaviors such as aggression and maturity

The many other causes of speech delay include bilingual children with phonological disorders autism spectrum disorders, childhood apraxia, Auditory processing disorder, prematurity, cognitive impairment and hearing loss. Broomfield and Dodd's (2004a) found out after survey that 6.4% of children who are perfectly normal showed speech difficulty while they lacked these disorders will often show early signs and are at times identified as "at risk" when the speech delay is diagnosed.

Muteness or mutism () is an inability to speak, often caused by a speech disorder, hearing loss, or surgery. Someone who is mute may be so due to the unwillingness to speak in certain social situations.

Acquired disorders result from brain injury, stroke or atrophy, many of these issues are included under the Aphasia umbrella.

Brain damage, for example, may result in various forms of aphasia if critical areas of the brain such as Broca's or Wernicke's area are damaged by lesions or atrophy as part of a dementia.

Expressive language disorder is a communication disorder in which there are difficulties with verbal and written expression. It is a specific language impairment characterized by an ability to use expressive spoken language that is markedly below the appropriate level for the mental age, but with a language comprehension that is within normal limits. There can be problems with vocabulary, producing complex sentences, and remembering words, and there may or may not be abnormalities in articulation.

As well as present speech production, very often, someone will have difficulty remembering things. This memory problem is only disturbing for speech; non-verbal or non-linguistically based memory will be unimpaired. An example of a child with expressive language disorder can be seen here.

Expressive language disorder affects work and schooling in many ways. It is usually treated by specific speech therapy, and usually cannot be expected to go away on its own.

Expressive language disorder can be further classified into two groups: developmental expressive language disorder and acquired expressive language disorder. Developmental expressive language disorder currently has no known cause, is first observed when a child is learning to talk, is more common in boys than girls, and is much more common than the acquired form of the disorder. Acquired expressive language disorder is caused by specific damage to the brain by a stroke, traumatic brain injury, or seizures.

Care must be taken to distinguish expressive language disorder from other communication disorders, sensory-motor disturbances, intellectual disability and/or environmental deprivation (see DSM-IV-TR criterion D). These factors affect a person's speech and writing to certain predictable extents, and with certain differences.

Careful diagnosis is also important because "atypical language development can be a secondary characteristic of other physical and developmental problems that may first manifest as language problems".

Those who are physically mute may have problems with the parts of the human body required for human speech (the esophagus, vocal cords, lungs, mouth, or tongue, etc.).

Trauma or injury to Broca's area, located in the left inferior frontal cortex of the brain, can cause muteness.

Mixed receptive-expressive language disorder (DSM-IV 315.32) is a communication disorder in which both the receptive and expressive areas of communication may be affected in any degree, from mild to severe. Children with this disorder have difficulty understanding words and sentences. This impairment is classified by deficiencies in expressive and receptive language development that is not attributed to sensory deficits, nonverbal intellectual deficits, a neurological condition, environmental deprivation or psychiatric impairments. Research illustrates that 2% to 4% of 5 year olds have mixed receptive-expressive language disorder. This distinction is made when children have issues in expressive language skills, the production of language, and when children also have issues in receptive language skills, the understanding of language. Those with mixed receptive-language disorder have a normal left-right anatomical asymmetry of the planum temporale and parietale. This is attributed to a reduced left hemisphere functional specialization for language. Taken from a measure of cerebral blood flow (SPECT) in phonemic discrimination tasks, children with mixed receptive-expressive language disorder do not exhibit the expected predominant left hemisphere activation. Mixed receptive-expressive language disorder is also known as receptive-expressive language impairment (RELI) or receptive language disorder.

Hyperlexia is a syndrome characterized by a child's precocious ability to read. It was initially identified by Norman E. Silberberg and Margaret C. Silberberg (1967), who defined it as the precocious ability to read words without prior training in learning to read, typically before the age of 5. They indicated that children with hyperlexia have a significantly higher word-decoding ability than their reading comprehension levels. Children with hyperlexia also present with an intense fascination for written material at a very early age.

Hyperlexic children are characterized by having average or above-average IQs, and word-reading ability well above what would be expected given their age. First named and scientifically described in 1967 (Silverberg and Silverberg), it can be viewed as a superability in which word recognition ability goes far above expected levels of skill. Some hyperlexics, however, have trouble understanding speech. Some experts believe that most, or perhaps all children with hyperlexia, lie on the autism spectrum. However, one expert, Darold Treffert, proposes that hyperlexia has subtypes, only some of which overlap with autism. Between 5 and 20 percent of autistic children have been estimated to be hyperlexic.

Hyperlexic children are often fascinated by letters or numbers. They are extremely good at decoding language and thus often become very early readers. Some hyperlexic children learn to spell long words (such as "elephant") before they are two years old and learn to read whole sentences before they turn three.

An fMRI study of a single child showed that hyperlexia may be the neurological opposite of dyslexia.

A communication disorder is any disorder that affects an individual's ability to comprehend, detect, or apply language and speech to engage in discourse effectively with others. The delays and disorders can range from simple sound substitution to the inability to understand or use one's native language.

Echolalia (also known as echologia or echophrasia) is defined as the unsolicited repetition of vocalizations made by another person (by the same person is called palilalia). In its profound form it is automatic and effortless. It is one of the echophenomena, closely related to echopraxia, the automatic repetition of movements made by another person; both are "subsets of imitative behavior" whereby sounds or actions are imitated "without explicit awareness". Echolalia may be an immediate reaction to a stimulus or may be delayed.

The word "echolalia" is derived from the Greek , meaning "echo" or "to repeat", and ("laliá") meaning "speech" or "talk" (of onomatopoeic origin, from the verb ("laléo"), meaning "to talk").

Auditory processing disorder (APD), also known as central auditory processing disorder (CAPD), is an umbrella term for a variety of disorders that affect the way the brain processes auditory information. Individuals with APD usually have normal structure and function of the outer, middle and inner ear (peripheral hearing). However, they cannot process the information they hear in the same way as others do, which leads to difficulties in recognizing and interpreting sounds, especially the sounds composing speech. It is thought that these difficulties arise from dysfunction in the central nervous system.

The American Academy of Audiology notes that APD is diagnosed by difficulties in one or more auditory processes known to reflect the function of the central auditory nervous system.

APD can affect both children and adults, although the actual prevalence is currently unknown. It has been suggested that males are twice as likely to be affected by the disorder as females, but there are no good epidemiological studies.

It has been discovered that APD and ADHD present overlapping symptoms. Below is a ranked order of behavioral symptoms that are most frequently observed in each disorder. Professionals evaluated the overlap of symptoms between the two disorders. The order below is of symptoms that are almost always observed. This chart proves that although the symptoms listed are different, it is easy to get confused between many of them.

There is a high rate of co-occurrence between AD/HD and CAPD. Research shows that 84% of children with APD have confirmed or suspected ADHD. Co-occurrence between ADHD and APD is 41% for children with confirmed diagnosis of ADHD, and 43% for children suspected of having ADHD.

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.

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.

Cluttering (also called tachyphemia or tachyphrasia) is a speech and communication disorder characterized by a rapid rate of speech, erratic rhythm, and poor syntax or grammar, making speech difficult to understand.

Children who demonstrate deficiencies early in their speech and language development are at risk for continued speech and language issues throughout later childhood. Similarly, even if these speech and language problems have been resolved, children with early language delay are more at risk for difficulties in phonological awareness, reading, and writing throughout their lives. Children with mixed receptive-expressive language disorder are often likely to have long-term implications for language development, literacy, behavior, social development, and even mental health problems. If suspected of having a mixed receptive-expressive language disorder, treatment is available from a speech therapist or pathologist. Most treatments are short term, and rely upon accommodations made within the environment, in order to minimize interfering with work or school. Programs that involve intervention planning that link verbal short term memory with visual/non-verbal information may be helpful for these children. In addition, approaches such as parent training for language stimulation and monitoring language through the "watch and see" method are recommended. The watch-and-see technique advises children with mixed receptive-expressive language disorder who come from stable, middle-class homes without any other behavioral, medical, or hearing problems should be vigilantly monitored rather than receive intervention. It is often the case that children do not meet the eligibility criteria established through a comprehensive oral language evaluation; and as a result, are not best suited for early intervention programs and require a different approach besides the "one size fits all" model.

In psychiatry, stilted speech or pedantic speech is communication characterized by situationally inappropriate formality. This formality can be expressed both through abnormal prosody as well as speech content that is "inappropriately pompous, legalistic, philosophical, or quaint". Often, such speech can act as evidence for autism spectrum disorder (ASD) or a thought disorder, a common symptom in schizophrenia or schizotypal personality disorder.

To diagnose stilted speech, researchers have previously looked for the following characteristics:

- speech conveying more information than necessary

- vocabulary and grammar expected from formal writing rather than conversational speech

- unneeded repetition or corrections

While literal and long-winded word content is often the most identifiable feature of stilted speech, such speech often displays irregular prosody, especially in resonance. Often, the loudness, pitch, rate, and nasality of pedantic speech vary from normal speech, resulting in the perception of pedantic or stilted speaking. For example, overly loud or high-pitched speech can come across to listeners as overly forceful while slow or nasal speech creates an impression of condescension.

These attributions, which are commonly found in patients with ASD, partially account for why stilted speech has been considered a diagnostic criteria for the disorder. Stilted speech, along with atypical intonation, semantic drift, terseness, and perseveration, are all qualities known to be commonly impaired during conversation with adolescents on the autistic spectrum. Often, stilted speech found in children with ASD will also be especially stereotypic or rehearsed.

Patients with schizophrenia are also known to experience stilted speech. This symptom is attributed to both an inability to access more commonly used words and a difficulty understanding pragmatics, or the relationship between language and context. However, stilted speech appears as less common symptom compared to a certain number of other symptoms of the psychosis (Adler "et al" 1999). This element of cognitive disorder is also exhibited as a symptom in the narcissistic personality disorder (Akhtar & Thomson 1982).

In its more common usage, "stilted speech" is a term used to describe overly-formal, unnatural-sounding speech.

In one paper, Darold Treffert proposes three types of hyperlexia. Specifically:

- Type 1: Neurotypical children who are very early readers.

- Type 2: Children on the autism spectrum who demonstrate very early reading as a splinter skill.

- Type 3: Very early readers who are not on the autism spectrum though there are some "autistic-like" traits and behaviours which gradually fade as the child gets older.

A different paper by Rebecca Williamson Brown, OD proposes only two types of hyperlexia. These are:

- Type 1: Hyperlexia marked by an accompanying language disorder.

- Type 2: Hyperlexia marked by an accompanying visual-spatial learning disorder.