Phonology is the science of speech sounds and speech patterns. It is a rule-based system. A phonological disorder occurs when a child has not learned the rules for combining sounds in words and creates their own. This affects classes of sounds rather than individual sounds.

For example, a child may voice all voiceless consonants ( "p, t, k" are pronounced as "b, d, g") or produce "back" sounds in the front of the mouth ("tup" for "cup", "tat" for "cat"). Although use of these patterns  (called phonological processes) is considered appropriate in young children, most phonological processes should resolve by age 5.

Phonological disorders often co-occur with articulation disorders. They may significantly impede speech intelligibility and can place children at risk for future reading and learning disabilities.