The warning signs of early speech delay are categorized into age related milestones, beginning at the age of 12 months and continuing through early adolescence.

At the age of 12 months, there is cause for concern if the child is not able to do the following:

- Using gestures such as waving good-bye and pointing at objects

- Practicing the use of several different consonant sounds

- Vocalizing or communicating needs

Between the ages of 15 and 18 months children are at a higher risk for speech delay if they are displaying the following:

- Not saying "momma" and "dada"

- Not reciprocating when told "no", "hello", and "bye"

- Does not have a one to three word vocabulary at 12 months and up to 15 words by 18 months

- Is unable to identify body parts

- Displaying difficulties imitating sounds and actions

- Shows preference to gestures over verbalization

Additional signs of speech delay after the age of 2 years and up to the age of 4 include the following:

- Inability to spontaneously produce words and phrases

- Inability to follow simple directions and commands

- Cannot make two word connections

- Lacks consonant sounds at the beginning or end of words

- Is difficult to understand by close family members

- Is not able to display the tasks of common household objects

- Is unable to form simple 2 to 3 word sentences

Speech and language impairment are basic categories that might be drawn in issues of communication involve hearing, speech, language, and fluency.

A speech impairment is characterized by difficulty in articulation of words. Examples include stuttering or problems producing particular sounds. Articulation refers to the sounds, syllables, and phonology produced by the individual. Voice, however, may refer to the characteristics of the sounds produced—specifically, the pitch, quality, and intensity of the sound. Often, fluency will also be considered a category under speech, encompassing the characteristics of rhythm, rate, and emphasis of the sound produced

A language impairment is a specific impairment in understanding and sharing thoughts and ideas, i.e. a disorder that involves the processing of linguistic information. Problems that may be experienced can involve the form of language, including grammar, morphology, syntax; and the functional aspects of language, including semantics and pragmatics

An individual can have one or both types of impairment. These impairments/disorders are identified by a speech and language pathologist.

Even though most speech sound disorders can be successfully treated in childhood, and a few may even outgrow them on their own, errors may sometimes persist into adulthood rather than only being not age appropriate. Such persisting errors are referred to as "residual errors" and may remain for life.

Selective mutism previously known as "elective mutism" is an anxiety disorder very common among young children, characterized by the inability to speak in certain situations. It should not to be confused with someone who is mute and cannot communicate due to physical disabilities. Selectively mute children are able to communicate in situations in which they feel comfortable. About 90% of children with this disorder have also been diagnosed with social anxiety. It is very common for symptoms to occur before the age of five and do not have a set time period. Not all children express the same symptoms. Some may stand motionless and freeze in specific social settings and have no communication.

Alalia is a disorder that refers to a delay in the development of speaking abilities in children. In severe cases, some children never learn how to speak. It is caused by illness of the child or the parents, the general disorders of the muscles, the shyness of the child or that the parents are close relatives.

Anarthria is a severe form of dysarthria. The coordination of movements of the mouth and tongue or the conscious coordination of the lungs are damaged.

Aphasia can rob all aspects of the speech and language. It is a damage of the cerebral centres of the language.

Aphonia is the inability to produce any voice. In severe cases the patient loses phonation. It is caused by the injury, paralysis, and illness of the larynx.

Conversion disorder can cause loss of speaking ability.

Feral children grow up outside of human society, and so usually struggle in learning any language.

Some people with autism never learn to speak.

Most intellectually disabled children learn to speak, but in the severe cases they can't learn speech (IQ 20-25). Children with Williams syndrome have good language skills with mean IQ 50. Children with Down syndrome often have impaired language and speech.

AOS and expressive aphasia (also known as Broca's aphasia) are commonly mistaken as the same disorder mainly because they often occur together in patients. Although both disorders present with symptoms such as a difficulty producing sounds due to damage in the language parts of the brain, they are not the same. The main difference between these disorders lies in the ability to comprehend spoken language; patients with apraxia are able to fully comprehend speech, while patients with aphasia are not always fully able to comprehend others' speech.

Conduction aphasia is another speech disorder that is similar to, but not the same as, apraxia of speech. Although patients who suffer from conduction aphasia have full comprehension of speech, as do AOS sufferers, there are differences between the two disorders. Patients with conduction aphasia are typically able to speak fluently, but they do not have the ability to repeat what they hear.

Similarly, dysarthria, another motor speech disorder, is characterized by difficulty articulating sounds. The difficulty in articulation does not occur due in planning the motor movement, as happens with AOS. Instead, dysarthria is caused by inability in or weakness of the muscles in the mouth, face, and respiratory system.

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.

Studies show that children diagnosed with speech delay are more likely to present with behavioral and social emotional problems both in childhood and as adults. Decreased receptive language, reading, and learning skills are common side effects for children that suffer from a speech delay and do not receive adequate intervention. Similar studies suggest that children with speech delays are more likely to have a difficult time communicating and bonding with peers, which could have negative effects on their psychosocial health later in life.

Apraxia of speech (AOS) is a neurogenic communication disorder affecting the motor programming system for speech production. Individuals with AOS demonstrate difficulty in speech production, specifically with sequencing and forming sounds. The Levelt model describes the speech production process in the following three consecutive stages: conceptualization, formulation, and articulation. According to the Levelt model, apraxia of speech would fall into the articulation region. The individual does not suffer from a language deficiency, but has difficulty in the production of language in an audible manner. Notably, this difficulty is limited to vocal speech, and does not affect sign-language production. The individual knows exactly what they want to say, but there is a disruption in the part of the brain that sends the signal to the muscle for the specific movement. Individuals with acquired AOS demonstrate hallmark characteristics of articulation and prosody (rhythm, stress or intonation) errors. Coexisting characteristics may include groping and effortful speech production with self-correction, difficulty initiating speech, abnormal stress, intonation and rhythm errors, and inconsistency with articulation.

Wertz et al., (1984) describe the following five speech characteristics that an individual with apraxia of speech may exhibit:

- Effortful trial and error with groping

- Self correction of errors

- Abnormal rhythm, stress and intonation

- Inconsistent articulation errors on repeated speech productions of the same utterance

- Difficulty initiating utterances

The following are brief definitions of several of the more prominent speech disorders:

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.

Individuals with PLI have particular trouble understanding the meaning of what others are saying, and they are challenged in using language appropriately to get their needs met and interact with others. Children with the disorder often exhibit:

- delayed language development

- aphasic speech (such as word search pauses, jargoning, word order errors, word category errors, verb tense errors)

- Stuttering or cluttering speech

- Repeating words or phrases

- difficulty with pronouns or pronoun reversal

- difficulty understanding questions

- difficulty understanding choices and making decisions.

- difficulty following conversations or stories. Conversations are "off-topic" or "one-sided".

- difficulty extracting the key points from a conversation or story; they tend to get lost in the details

- difficulty with verb tenses

- difficulty explaining or describing an event

- tendency to be concrete or prefer facts to stories

- difficulty understanding satire or jokes

- difficulty understanding contextual cues

- difficulty in reading comprehension

- difficulty with reading body language

- difficulty in making and maintaining friendships and relationships because of delayed language development.

- difficulty in distinguishing offensive remarks

- difficulty with organizational skills

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".

Speech disorders or speech impediments are a type of communication disorder where 'normal' speech is disrupted. This can mean stuttering, lisps, etc. Someone who is unable to speak due to a speech disorder is considered mute.

Classifying speech into normal and disordered is more problematic than it first seems. By a strict classification, only 5% to 10% of the population has a completely normal manner of speaking (with respect to all parameters) and healthy voice; all others suffer from one disorder or another.

There are three different levels of classification when determining the magnitude and type of a speech disorders and the proper treatment or therapy:

1. Sounds the patient can produce

1. Phonemic – can be produced easily; used meaningfully and constructively

2. Phonetic – produced only upon request; not used consistently, meaningfully, or constructively; not used in connected speech

2. Stimulate sounds

1. Easily stimulated

2. Stimulate after demonstration and probing (i.e. with a tongue depressor)

3. Cannot produce the sound

1. Cannot be produced voluntarily

2. No production ever observed

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).

If assessed on the Wechsler Adult Intelligence Scale, for instance, symptoms of mixed receptive-expressive language disorder may show as relatively low scores for Information, Vocabulary and Comprehension (perhaps below the 25th percentile). If a person has difficulty with specific types of concepts, for example spatial terms, such as 'over', 'under', 'here' and 'there', they may also have difficulties with arithmetic, understanding word problems and instructions, or difficulties using words at all.

They may also have a more general problem with words or sentences, both comprehension and orally. Some children will have issues with pragmatics - the use of language in social contexts as well; and therefore, will have difficulty with inferring meaning. Furthermore, they have severe impairment of spontaneous language production and for this reason, they have difficulty in formulating questions. Generally, children will have trouble with morphosyntax, which is word inflections. These children have difficulty understanding and applying grammatical rules, such as endings that mark verb tenses (e.g. -"ed"), third-person singular verbs (e.g. I "think", he "thinks"), plurals (e.g. -"s"), auxiliary verbs that denote tenses (e.g. "was" running, "is" running), and with determiners ("the, a"). Moreover, children with mixed receptive-expressive language disorders have deficits in completing two cognitive operations at the same time and learning new words or morphemes under time pressure or when processing demands are high. These children also have auditory processing deficits in which they process auditory information at a slower rate and as a result, require more time for processing.

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.

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

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.

Speech is the act of articulating sounds, and this can be impaired for all kinds of reasons – a structural problem such as cleft lip and cleft palate, a neurological problem affecting motor control of the speech apparatus Dysarthria, or inability to perceive distinctions between sounds because of Hearing loss. Some distortions of speech sounds, such as a Lisp, are commonly seen in young children. These misarticulations should not be confused with language problems, which involve the ability to select and combine linguistic elements to express meanings, and the ability to comprehend meanings.

Although speech disorders can be distinguished from language disorders, they can also co-occur. When a child fails to produce distinctions between speech sounds for no obvious reason, this is typically regarded as a language problem affecting the learning of phonological contrasts. The classification of and terminology for disorders of speech sound production is a subject of considerable debate. In practice, even for those with specialist skills, it is not always easy to distinguish between phonological disorders and other types of speech production problem.

Speech sound disorder (SSD) is a general term for problems with speech production arising from any cause.

Speech Sound Disorders of unknown cause that are not accompanied by other language problems are a relatively common reason for young children to be referred to speech-language therapy (speech-language pathology). These often resolve by around 4-5 years of age with specialist intervention, and so would not meet criteria for DLD. Where such problems continue beyond 5 years of age, they are usually accompanied by problems in broader language domains and have a poorer prognosis, so a diagnosis of DLD with SSD is then appropriate.

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.

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.

Studies show that low receptive and expressive language at young ages was correlated to increased autism symptom severity in children in their early school years. Below is a chart depicting language deficits of children on the Autistic Spectrum. This table indicates the lower levels of language processing, receptive/expressive disorders, which is more severe in children with autism. When autistic children speak, they are often difficult to understand, their language is sparse and dysfluent, they speak in single, uninflected words or short phrases, and their supply of words is severely depleted. This leads to limited vocabulary while also having deficits in verbal short term memory.

Developmental language disorder (DLD) is identified when a child has problems with language development that continue into school age and beyond. The language problems have a significant impact on everyday social interactions or educational progress, and occur in the absence of autism spectrum disorder, intellectual disability or a known biomedical condition. The most obvious problems are difficulties in using words and sentences to express meanings, but for many children, understanding of language (receptive language) is also a challenge, although this may not be evident unless the child is given a formal assessment.

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