Phonology is the sound system of language. The phonology of language tells us how sounds fit together in words. Children who have phonological processing disorders have not learned the rules for how sounds fit together to make words, and use certain processes to simplify words. Phonological processing disorders are related to language, and are now seen as a language-based disorder. Children with phonological processing disorders are frequently unintelligible; often, their parents are the only ones who can understand them, and even they have difficulties. Children with these disorders are at a very high risk for later reading and learning disabilities, and should be treated with intensive speech therapy as soon as they are diagnosed, as early as age 3.

A phonological processing disorder is most commonly diagnosed using the Assessment of Phonological Processes by Barbara Hodson. This test analyzes (by hand, or through a computer program) the patterns that a child is producing as they say 50 words. A phonological analysis can also be completed informally. There are other tests for phonology available, but the Hodson is the most widely used. There are many different phonological processes which SLPs see and treat. One of the most common is called "cluster reduction". Children who use this process will take a sound blend (like [bl] [sp] or [tr]) and omit one of the sounds: "blue" becomes "boo", "spoon" becomes "poon", and "tree" becomes "ti". Another common process is called "velar fronting". Children who use this process substitute sounds produced in the front of the mouth (t, d, n) for sounds produced in the back of the throat (k, g). In this instance, "gun" becomes "dun", "car" becomes "tar", "go" becomes "do" and "can" becomes "tan".