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

The DSM-5 categorizes SCD as a communication disorder within the domain of neurodevelopmental disorders, listed alongside other disorders of speech and language which typically manifest in early childhood. The DSM-5 diagnostic criteria for social (pragmatic) communication disorder is as follows:

- A. Persistent difficulties in the social use of verbal and nonverbal communication as manifested by all of the following:

1. Deficits in using communication for social purposes, such as greeting and sharing information, in a manner that is appropriate for the social context.

2. Impairment of the ability to change communication to match context or the needs of the listener, such as speaking differently in a classroom than on a playground, talking differently to a child than to an adult, and avoiding use of overly formal language.

3. Difficulties following rules for conversation and storytelling, such as taking turns in conversation, rephrasing when misunderstood, and knowing how to use verbal and nonverbal signals to regulate interaction.

4. Difficulties understanding what is not explicitly stated (e.g., making inferences) and nonliteral or ambiguous meanings of language (e.g., idioms, humor, metaphors, multiple meanings that depend on the context for interpretation).

- B. The deficits result in functional limitations in effective communication, social participation, social relationships, academic achievement, or occupational performance, individually or in combination.

- C. The onset of symptoms is in the early developmental period (but deficits may not become fully manifest until social communication demands exceed limited capacities).

- D. The symptoms are not attributable to another medical or neurological condition or to low abilities in the domains of word structure and grammar, and are not better explained by autism spectrum disorder, intellectual disability (intellectual developmental disorder), global developmental delay, or another mental disorder.

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.

The word hyperlexia is derived from the Greek terms "hyper" ("over") and "léxis" ("diction", "word").

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".

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.

Nonverbal autism is a subset of autism where the subject is unable to speak. While most autistic children eventually begin to speak, there is a significant minority who will remain nonverbal.

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

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

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

The following diagnoses are included in the communication disorders:

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

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

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

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

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

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.

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.

The term developmental language disorder (DLD) was endorsed in a consensus study involving a panel of experts (CATALISE Consortium) in 2017... The study was conducted in response to concerns that a wide range of terminology was used in this area, with the consequence that there was poor communication, lack of public recognition, and in some cases children were denied access to services. Developmental language disorder is a subset of language disorder, which is itself a subset of the broader category of speech, language and communication needs (SLCN).

The terminology for children’s language disorders has been extremely wide-ranging and confusing, with many labels that have overlapping but not necessarily identical meanings. In part this confusion reflected uncertainty about the boundaries of DLD, and the existence of different subtypes. Historically, the terms ‘’developmental dysphasia’’ or ‘’developmental aphasia’’ were used to describe children with the clinical picture of DLD. These terms have, however, largely been abandoned, as they suggest parallels with adult acquired aphasia. This is misleading, as DLD is not caused by brain damage.

Although the term DLD has been used for many years, it has been less common than the term Specific language impairment (SLI), which has been widely adopted, especially in North America. The definition of SLI overlaps with DLD, but was rejected by the CATALISE panel because it was seen as overly restrictive in implying that the child had relatively pure problems with language in the absence of any other impairments. Children with such selective problems are relatively rare, and there is no evidence that they respond differently to intervention, or have different causal factors, from other children with language problems.

UK speech and language therapists widely use the term Speech, Language and Communication Needs (SLCN), but this is far broader than DLD, and includes children with speech and language difficulties arising from a wide range of causes.

The question of whether to refer to children's language problems as ‘disorder’ was a topic of debate among the CATALISE consortium, but the conclusion was that ‘disorder’ conveyed the serious nature and potential consequences of persistent language deficits. It is also parallel with other neurodevelopmental conditions and consistent with diagnostic frameworks such as DSM5 and ICD-11. Where there are milder or more transient difficulties, language difficulties may be a more appropriate term.

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).

Although hyperlexic children usually learn to read in a non-communicative way, several studies have shown that they can acquire reading comprehension and communicative language after the onset of hyperlexia. They follow a different developmental trajectory relative to neurotypical individuals, with milestones being acquired in a different order. Despite hyperlexic children's precocious reading ability, they may struggle to communicate. Often, hyperlexic children will have a precocious ability to read but will learn to speak only by rote and heavy repetition, and may also have difficulty learning the rules of language from examples or from trial and error, which may result in social problems. Their language may develop using echolalia, often repeating words and sentences. Often, the child has a large vocabulary and can identify many objects and pictures, but cannot put their language skills to good use. Spontaneous language is lacking and their pragmatic speech is delayed. Hyperlexic children often struggle with Who? What? Where? Why? and How? questions. Between the ages of 4 and 5 years old, many children make great strides in communicating.

The social skills of a child with hyperlexia often lag tremendously. Hyperlexic children often have far less interest in playing with other children than do their peers.

In early childhood, symptoms that correlate with a later diagnosis of dyslexia include delayed onset of speech and a lack of phonological awareness, as well as being easily distracted by background noise. A common myth closely associates dyslexia with mirror writing and reading letters or words backwards. These behaviors are seen in many children as they learn to read and write, and are not considered to be defining characteristics of dyslexia.

School-age children with dyslexia may exhibit signs of difficulty in identifying or generating rhyming words, or counting the number of syllables in words – both of which depend on phonological awareness. They may also show difficulty in segmenting words into individual sounds or may blend sounds when producing words, indicating reduced phonemic awareness. Difficulties with word retrieval or naming things is also associated with dyslexia. People with dyslexia are commonly poor spellers, a feature sometimes called dysorthographia or dysgraphia, which depends on orthographic coding.

Problems persist into adolescence and adulthood and may accompany difficulties with summarizing stories, memorization, reading aloud, or learning foreign languages. Adults with dyslexia can often read with good comprehension, though they tend to read more slowly than others without a learning difficulty and perform worse in spelling tests or when reading nonsense words – a measure of phonological awareness.

Although textbooks draw clear boundaries between different neurodevelopmental disorders, there is much debate about overlaps between them. Many children with SLI meet diagnostic criteria for developmental dyslexia, and others have features of autism.

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.

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

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

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

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.

The orthographic complexity of a language directly impacts how difficult learning to read the language is. English and French have comparatively "deep" phonemic orthographies within the Latin alphabet writing system, with complex structures employing spelling patterns on several levels: letter-sound correspondence, syllables, and morphemes. Languages such as Spanish, Italian and Finnish have mostly alphabetic orthographies, which primarily employ letter-sound correspondence – so-called shallow orthographies – which for dyslexics makes them easier to learn. Logographic writing systems, such as Chinese characters, have extensive symbol use, and pose problems for dyslexic learners.

Deficits in any area of information processing can manifest in a variety of specific learning disabilities. It is possible for an individual to have more than one of these difficulties. This is referred to as comorbidity or co-occurrence of learning disabilities. In the UK, the term "dual diagnosis" is often used to refer to co-occurrence of learning difficulties.

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.

SLI is defined purely in behavioural terms: there is no biological test for SLI. There are three points that need to be met for a diagnosis of SLI:

- The child has language difficulties that interfere with daily life or academic progress

- Other causes are excluded: the problems cannot be explained in terms of hearing loss, general developmental delay, autism, or physical difficulty in speaking

- Performance on a standardized language test (see Assessment, below) is significantly below age level

There is considerable variation in how this last criterion is implemented. Tombin et al. (1996) proposed the EpiSLI criterion, based on five composite scores representing performance in three domains of language (vocabulary, grammar, and narration) and two modalities (comprehension and production). Children scoring in the lowest 10% on two or more composite scores are identified as having language disorder.

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.