A disruption of the onward flow of speech. The individual knows what he or she wants to say, but temporarily loses the ability to produce fluent speech. There is a silent or audible involuntary repetition/prolongation of sounds, syllables or words, sometimes accompanied by accessory behaviours, or speech-related struggle. Usually there is an accompanying emotional state, involving excitement, tension, fear or embarrassment.

Generally stammering begins between the onset of speech and puberty, mostly between 2–5 years of age. Acquired stammering at a later age is rare and usually results from brain damage. Most stammering children recover with little or no therapy. Stammerers have not been shown to have different personalities from non-stammerers, but some show minimal differences from fluent speakers in the way their brains process spoken language.

The risk of stammering among first-degree relatives of stammerers is more than three times the population risk. As there are identical twins who differ in this way, environmental factors must also be important. There are relatively large numbers of stammerers in highly competitive societies, where status and prestige are important and high standards of speech competence are valued. Stammerers can learn how to speak in a different way (fluency-shaping techniques). This involves teaching them to slow down the rate of speech, to start each word gently and to keep up a constant flow with correct joins between words and sentences. Once targets have been achieved while at the clinic, a series of speech assignments is set for outside the clinic. In these assignments, and initially in everyday situations, the fluency-enhancing techniques have to be used conscientiously. Gradually speech is shaped towards normality, requiring less and less effort. Therapy may also include some work on attitude change (i.e. helping the client to see him or herself as a fluent speaker), and possibly general communicative skills training.