Primary stuttering behaviors are the overt, observable signs of speech disfluencies, including repeating sounds, syllables, words or phrases, silent blocks and prolongation of sounds. These differ from the normal dysfluencies found in all speakers in that stuttering dysfluencies may last longer, occur more frequently, and are produced with more effort and strain. Stuttering dysfluencies also vary in quality: common dysfluencies tend to be repeated movements, fixed postures, or superfluous behaviors. Each of these three categories is composed of subgroups of stutters and dysfluencies.

- Repeated movements

- Part-word repetition—a single segment of a word is repeated (for example: "s-s-stuttering!") or a part of a word which is still a full syllable such as "un—un—under the..." and "o—o—open".

- Incomplete syllable repetition—an incomplete syllable is repeated, such as a consonant without a vowel, for example, "c—c—c—cold".

- Whole-word repetition—a whole word, or more than one word is repeated, such as "I know—I know—I know a lot of information.".

- Fixed postures

- Prolongation—prolongation of a sound occurs such as "mmmmmmmmmom".

- Block—such as a block of speech or a tense pause where nothing is said despite efforts.

- Superfluous behaviors

- Interjections—this includes an interjection such as an unnecessary "uh" or "um" as well as revisions, such as going back and correcting one's initial statements such as "I—My girlfriend...", where the "I" has been corrected to the word "my".

- Secondary characteristics—these are visible or audible speech behaviors, such as lip smacking, throat clearing, head thrusting, etc., usually representing an effort to break through or circumvent a block or stuttering loop.

Stuttering, also known as stammering, is a speech disorder in which the flow of speech is disrupted by involuntary repetitions and prolongations of sounds, syllables, words or phrases as well as involuntary silent pauses or blocks in which the person who stutters is unable to produce sounds. The term "stuttering" is most commonly associated with involuntary sound repetition, but it also encompasses the abnormal hesitation or pausing before speech, referred to by people who stutter as "blocks", and the prolongation of certain sounds, usually vowels or semivowels. According to Watkins et al., stuttering is a disorder of "selection, initiation, and execution of motor sequences necessary for fluent speech production." For many people who stutter, repetition is the primary problem. The term "stuttering" covers a wide range of severity, encompassing barely perceptible impediments that are largely cosmetic to severe symptoms that effectively prevent oral communication. In the world, approximately four times as many men as women stutter, encompassing 70 million people worldwide, or about 1% of the world's population.

The impact of stuttering on a person's functioning and emotional state can be severe. This may include fears of having to enunciate specific vowels or consonants, fears of being caught stuttering in social situations, self-imposed isolation, anxiety, stress, shame, being a possible target of bullying (especially in children), having to use word substitution and rearrange words in a sentence to hide stuttering, or a feeling of "loss of control" during speech. Stuttering is sometimes popularly seen as a symptom of anxiety, but there is actually no direct correlation in that direction (though as mentioned the inverse can be true, as social anxiety may actually develop in individuals as a result of their stuttering).

Stuttering is generally not a problem with the physical production of speech sounds or putting thoughts into words. Acute nervousness and stress do not cause stuttering, but they can trigger stuttering in people who have the speech disorder, and living with a stigmatized disability can result in anxiety and high allostatic stress load (chronic nervousness and stress) that reduce the amount of acute stress necessary to trigger stuttering in any given person who stutters, exacerbating the problem in the manner of a positive feedback system; the name 'stuttered speech syndrome' has been proposed for this condition. Neither acute nor chronic stress, however, itself creates any predisposition to stuttering.

The disorder is also "variable", which means that in certain situations, such as talking on the telephone or in a large group, the stuttering might be more severe or less, depending on whether or not the stutterer is self-conscious about their stuttering. Stutterers often find that their stuttering fluctuates and that they have "good" days, "bad" days and "stutter-free" days. The times in which their stuttering fluctuates can be random. Although the exact etiology, or cause, of stuttering is unknown, both genetics and neurophysiology are thought to contribute. There are many treatments and speech therapy techniques available that may help decrease speech disfluency in some people who stutter to the point where an untrained ear cannot identify a problem; however, there is essentially no cure for the disorder at present. The severity of the person's stuttering would correspond to the amount of speech therapy needed to decrease disfluency. For severe stuttering, long-term therapy and hard work is required to decrease disfluency.

There are three significant features that differentiate DVD/CAS from other childhood speech sound disorders. These features are:

- "Inconsistent errors on consonants and vowels in repeated productions of syllables and words

- Lengthened coarticulatory transitions between sounds and syllables

- Inappropriate prosody, especially in the realization of lexical or phrasal stress"

Even though DVD/CAS is a "developmental" disorder, it will not simply disappear when children grow older. Children with this disorder do not follow typical patterns of language acquisition and will need treatment in order to make progress.

Stuttering is often misapplied as a common term referring to any dysfluency. It is also often incorrectly applied to normal dysfluency rather than dysfluency from a disorder. Cluttered speech is exhibited by normal speakers, and is often referred to as stuttering. This is especially true when the speaker is nervous, where nervous speech more closely resembles cluttering than stuttering.

Cluttering is sometimes confused with stuttering. Both communication disorders break the normal flow of speech, but they are distinct. A stutterer has a coherent pattern of thoughts, but may have a difficult time vocally expressing those thoughts; in contrast, a clutterer has no problem putting thoughts into words, but those thoughts become disorganized during speaking. Cluttering affects not only speech, but also thought patterns, writing, typing, and conversation.

Stutterers are usually dysfluent on initial sounds, when beginning to speak, and become more fluent towards the ends of utterances. In contrast, clutterers are most clear at the start of utterances, but their speaking rate increases and intelligibility decreases towards the end of utterances.

Stuttering is characterized by struggle behavior, such as overtense speech production muscles. Cluttering, in contrast, is effortless. Cluttering is also characterized by slurred speech, especially dropped or distorted /r/ and /l/ sounds; and monotone speech that starts loud and trails off into a murmur.

A clutterer described the feeling associated with a clutter as:

Cluttering is a speech and communication disorder that has also been described as a fluency disorder.

It is defined as:"Cluttering is a fluency disorder characterized by a rate that is perceived to be abnormally rapid, irregular, or both for the speaker (although measured syllable rates may not exceed normal limits). These rate abnormalities further are manifest in one or more of the following symptoms: (a) an excessive number of disfluencies, the majority of which are not typical of people with stuttering; (b) the frequent placement of pauses and use of prosodic patterns that do not conform to syntactic and semantic constraints; and (c) inappropriate (usually excessive) degrees of coarticulation among sounds, especially in multisyllabic words".

Developmental verbal dyspraxia (DVD), also known as childhood apraxia of speech (CAS) and developmental apraxia of speech (DAS), is when children have problems saying sounds, syllables, and words. This is not because of muscle weakness or paralysis. The brain has problems planning to move the body parts (e.g., lips, jaw, tongue) needed for speech. The child knows what they want to say, but their brain has difficulty coordinating the muscle movements necessary to say those words. The exact cause of this disorder is unknown. Some observations suggest a genetic cause of DVD, as many with the disorder have a family history of communication disorders. There is no cure for DVD, but with appropriate, intensive intervention, people with this motor speech disorder can improve significantly.

Developmental verbal dyspraxia (DVD) is a type of ideational dyspraxia, causing speech and language impairments. This is the favoured term in the UK; however, it is also sometimes referred to as articulatory dyspraxia, and in the United States the usual term is childhood apraxia of speech (CAS).

Key problems include:

- Difficulties controlling the speech organs.

- Difficulties making speech sounds

- Difficulty sequencing sounds

- Within a word

- Forming words into sentences

- Difficulty controlling breathing, suppressing salivation and phonation when talking or singing with lyrics.

- Slow language development

Fine-motor problems can cause difficulty with a wide variety of other tasks such as using a knife and fork, fastening buttons and shoelaces, cooking, brushing one's teeth, styling one's hair, shaving, applying cosmetics, opening jars and packets, locking and unlocking doors, and doing housework.

Difficulties with fine motor co-ordination lead to problems with handwriting, which may be due to either ideational or ideo-motor difficulties.

Problems associated with this area may include:

- Learning basic movement patterns.

- Developing a desired writing speed.

- Establishing the correct pencil grip

- The acquisition of graphemes – e.g. the letters of the Latin alphabet, as well as numbers.

Symptoms of developmental dysfluency include the repetition of sounds or pauses between words. These symptoms have generally been noted within youngsters from 18 months to 5 years of age. This may persist for weeks or months but eventually disappears due to the maturation of the child's nervous system.

ex.: "Daddy, I was, I was, um, um, I was..."

Instances like the aforementioned example indicate that the child is learning to use language. On the other hand, children with stuttering disorder, will likely repeat sounds or one-syllable words three or more times. They may also prolong sounds for two or more seconds. In comparison, stuttering can be seen as a process where a word appears to become "stuck," and the person may grimace, jerk the head or neck as he struggles to overcome the stutter.

Children with normal dysfluency tend to have stuttering difficulties that come and go. Generally this is during preschool years and the problem normally ceases altogether by the time a child starts school.

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".

Dysarthria is the reduced ability to motor plan volitional movements needed for speech production as the result of weakness/paresis and/or paralysis of the musculature of the oral mechanism needed for respiration, phonation, resonance, articulation, and/or prosody.

Language delay is a failure in children to develop language abilities on the usual age appropriate for their developmental timetable. Language delay is distinct from speech delay, in which the development of the mechanical and motor aspects of speech production is delayed.

Oral communication is a two-stage process. The first stage is to encode the message into a set of words and sentence structures that convey the required meaning, i.e. into language. In the second stage, language is translated into motor commands that control the articulators (organs and structures such as the lungs, vocal cords, mouth, tongue, teeth, etc.), thereby creating speech, i.e. orally-expressed language.

Because language and speech are independent, 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. However a child with a language delay typically has not yet been able to use language to formulate material to speak; he or she is therefore likely to have a delay in speech as well.

Language delay is commonly divided into "receptive" and "expressive" categories. Receptive language refers to the process of understanding what is said to the subject. Expressive language refers to the use of words and sentences to communicate messages to others. Both categories are essential to effective communication.

Language disorders or language impairments are disorders that involve the processing of linguistic information. Problems that may be experienced can involve grammar (syntax and/or morphology), semantics (meaning), or other aspects of language. These problems may be receptive (involving impaired language comprehension), expressive (involving language production), or a combination of both. Examples include specific language impairment and aphasia, among others. Language disorders can affect both spoken and written language, and can also affect sign language; typically, all forms of language will be impaired.

Current data indicates that 7% of young children display language disorder, with boys being diagnosed twice as much as girls.

Preliminary research on potential risk factors have suggested biological components, such as low-birth weight, prematurity, general birth complications, and male gender, as well as family history and low parental education can increase the chance of developing language disorders.

For children with phonological and expressive language difficulties, there is evidence supporting speech and language therapy. However, the same therapy is shown to be much less effective for receptive language difficulties. These results are consistent with the poorer prognosis for receptive language impairments that are generally accompanied with problems in reading comprehension.

Note that these are distinct from speech disorders, which involve difficulty with the act of speech production, but not with language.

Language disorders tend to manifest in two different ways: receptive language disorders (where one cannot properly comprehend language) and expressive language disorders (where one cannot properly communicate their intended message).

Examples of disorders that may include or create challenges in language and communication and/or may co-occur with the above disorders:

- autism spectrum disorder - autistic disorder (also called "classic" autism), pervasive developmental disorder, and Asperger syndrome – developmental disorders that affect the brain's normal development of social and communication skills.

- expressive language disorder – affects speaking and understanding where there is no delay in non-verbal intelligence.

- mixed receptive-expressive language disorder – affects speaking, understanding, reading and writing where there is no delay in non-verbal intelligence.

- specific language impairment – a language disorder that delays the mastery of language skills in children who have no hearing loss or other developmental delays. SLI is also called developmental language disorder, language delay, or developmental dysphasia.

There are two types of Apraxia. Developmental (or Childhood Apraxia of speech) or acquired Apraxia. Childhood apraxia of speech (CAS) is a neurological childhood speech sound disorder that involves impaired precision and consistency of movements required for speech production without any neuromuscular deficits (ASHA, 2007a, Definitions of CAS section, para. 1). Both are the inability to plan volitional motor movements for speech production in the absence of muscular weakness. Apraxia is not a result of sensory problems, or physical issues with the articulatory structures themselves, simply the way the brain plans to move them.

Language delay is a risk factor for other types of developmental delay, including social, emotional, and cognitive delay. Some children may grow out of these deficits, even coming to excel where they once lagged, while others do not. One particularly common result of language delay is delayed or inadequate acquisition of reading skills. Reading depends upon an ability to code and decode script (i.e., match speech sounds with symbols, and vice versa). If a child is still struggling to master language and speech, it is very difficult to learn another level of complexity (writing). Thus, it is crucial that children have facility with language to be successful readers.

Neuroscientist Steven Pinker postulates that a certain form of language delay may be associated with exceptional and innate analytical prowess in some individuals, such as Albert Einstein, Richard Feynman and Edward Teller.

Developmental dysfluency is considered a normal part of childhood development. About twenty-five percent of children experience some loss in fluency.

Aphasia is loss of the ability to produce or comprehend language. There are acute aphasias which result from stroke or brain injury, and primary progressive aphasias caused by progressive illnesses such as dementia.

- Acute aphasias

- Expressive aphasia also known as Broca's aphasia, expressive aphasia is a non-fluent aphasia that is characterized by damage to the frontal lobe region of the brain. A person with expressive aphasia usually speaks in short sentences that make sense but take great effort to produce. Also, a person with expressive aphasia understands another person's speech but has trouble responding quickly.

- Receptive aphasia also known as Wernicke's aphasia, receptive aphasia is a fluent aphasia that is categorized by damage to the temporal lobe region of the brain. A person with receptive aphasia usually speaks in long sentences that have no meaning or content. People with this type of aphasia often have trouble understanding other's speech and generally do not realize that they are not making any sense.

- Conduction aphasia

- Anomic aphasia

- Global aphasia

- Primary progressive aphasias

- Progressive nonfluent aphasia

- Semantic dementia

- Logopenic progressive aphasia

Receptive language disorders can be acquired or developmental (most often the latter). When developmental, difficulties in spoken language tend to occur before three years of age. Usually such disorders are accompanied by expressive language disorders.

However, unique symptoms and signs of a receptive language disorder include: struggling to understand meanings of words and sentences, struggling to put words in proper order, and inability to follow verbal instruction.

Treatment options include: language therapy, special education classes for children at school, and a psychologist if accompanying behavioral problems are present.

Dyscalculia is difficulty in learning or comprehending arithmetic, such as difficulty in understanding numbers, learning how to manipulate numbers, and learning facts in mathematics. It is generally seen as a specific developmental disorder.

Dyscalculia can occur in people from across the whole IQ range – often higher than average – along with difficulties with time, measurement, and spatial reasoning. Estimates of the prevalence of dyscalculia range between 3 and 6% of the population. In 2004, it was reported that a quarter of children with dyscalculia had ADHD.

In 2015, it was established that 11% of children with dyscalculia also have ADHD. Dyscalculia has also been associated with people who have Turner syndrome and people who have spina bifida.

Mathematical disabilities can occur as the result of some types of brain injury, in which case the proper term, acalculia, is to distinguish it from dyscalculia which is of innate, genetic or developmental origin.

Although many researchers believe dyscalculia to be a persistent disorder, evidence on the persistence of dyscalculia remains mixed. For instance, in a study done by Mazzocco and Myers (2003), researchers evaluated children on a slew of measures and selected their most consistent measure as their best diagnostic criterion: a stringent 10th-percentile cut-off on the TEMA-2. Even with their best criterion, they found dyscalculia diagnoses for children longitudinally did not persist; only 65% of students who were ever diagnosed over the course of four years were diagnosed for at least two years. The percentage of children who were diagnosed in two consecutive years was further reduced. It is unclear whether this was the result of misdiagnosed children improving in mathematics and spatial awareness as they progressed as normal, or that the subjects who showed improvement were accurately diagnosed, but exhibited signs of a non-persistent learning disability.

Children with a tic disorder may exhibit the following symptoms:

- overwhelming urge to make movement

- jerking of arms

- clenching of fists

- excessive eye blinking

- shrugging of shoulders

- kicking

- raising eyebrows

- flaring of nostrils

- production of repetitive noises such as grunting, clicking, moaning, snorting, squealing, or throat clearing

Children with Attention Deficit and Disruptive Behavior Disorders may show the following symptoms:

- impulsivity or distractibility

- difficulty socializing

- aggressive behavior

- difficulty following rules or directions

- difficulty completing a task

- problems at school

- frustration

- alcohol or drug use

Prosopagnosia, also called face blindness, is a cognitive disorder of face perception in which the ability to recognize familiar faces, including one's own face (self-recognition), is impaired, while other aspects of visual processing (e.g., object discrimination) and intellectual functioning (e.g., decision making) remain intact. The term originally referred to a condition following acute brain damage (acquired prosopagnosia), but a congenital or developmental form of the disorder also exists, which may affect up to 2.5% of the United States population. The specific brain area usually associated with prosopagnosia is the fusiform gyrus, which activates specifically in response to faces. The functionality of the fusiform gyrus allows most people to recognize faces in more detail than they do similarly complex inanimate objects. For those with prosopagnosia, the new method for recognizing faces depends on the less-sensitive object recognition system. The right hemisphere fusiform gyrus is more often involved in familiar face recognition than the left. It remains unclear whether the fusiform gyrus is only specific for the recognition of human faces or if it is also involved in highly trained visual stimuli.

There are two types of prosopagnosia: acquired and congenital (developmental). Acquired prosopagnosia results from occipito-temporal lobe damage and is most often found in adults. This is further subdivided into apperceptive and associative prosopagnosia. In congenital prosopagnosia, the individual never adequately develops the ability to recognize faces.

Though there have been several attempts at remediation, no therapies have demonstrated lasting real-world improvements across a group of prosopagnosics. Prosopagnosics often learn to use "piecemeal" or "feature-by-feature" recognition strategies. This may involve secondary clues such as clothing, gait, hair color, skin color, body shape, and voice. Because the face seems to function as an important identifying feature in memory, it can also be difficult for people with this condition to keep track of information about people, and socialize normally with others. Prosopagnosia has also been associated with other disorders that are associated with nearby brain areas: left hemianopsia (loss of vision from left side of space, associated with damage to the right occipital lobe), achromatopsia (a deficit in color perception often associated with unilateral or bilateral lesions in the temporo-occipital junction) and topographical disorientation (a loss of environmental familiarity and difficulties in using landmarks, associated with lesions in the posterior part of the parahippocampal gyrus and anterior part of the lingual gyrus of the right hemisphere). It is from the Greek: "prosopon" = "face" and "agnosia" = "not knowing".

The tenth revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10) has four categories of specific developmental disorder: specific developmental disorders of speech and language, specific developmental disorders of scholastic skills, specific developmental disorder of motor function, and mixed specific developmental disorder.