"Developmental Phonological Disorders, also known as phonological disability or phonological disorders, are a group of language disorders that affect children’s ability to develop easily understood speech by the time they are four years old, and, in some cases, their ability to learn to read and spell. Therefore, Phonological disorders involve a difficulty in learning and organizing all the sounds needed for clear speech, reading and spelling". (Bowen, C. (1998). Developmental phonological disorders: A practical guide for families and teachers. Melbourne: The Australian Council for Educational Research Ltd.)

Individuals with this Communication Disorder of childhood demonstrate impairment in their ability to produce sounds as expected for their developmental level.
Failure to use developmentally expected speech sounds that are appropriate for age and dialect  (e.g., errors in sound production, use, representation, or organization such as, but not limited to, substitutions of one sound for another [use of /t/ for target /k/ sound] or omissions of sounds such as final consonants).

The difficulties in speech sound production interfere with academic or occupational achievement or with social communication.
"Some children with developmental phonological disorders have other speech and language difficulties such as immature grammar and syntax, stuttering or word-retrieval difficulties. However, many of them just have a 'pure' developmental phonological disorder, involving:
A problem with speech clarity in the preschool years, with no subsequent reading and spelling problems, or
A problem with speech clarity in the pre-school years, and, in the early school years, difficulty learning to read, and difficulties with reading comprehension, or
Speech and reading problems as described above, plus difficulty with spelling, or
Speech and spelling problems (i.e., no reading difficulties), or
Speech clarity problems in the pre-school years, and difficulties with written expression in primary school."

(Bowen, C. (1998). Developmental phonological