Stuttering, also known as stammering, is a speech disorder in which the flow of speech is disrupted by involuntary repetitions and prolongations of sounds, syllables, words or phrases, and involuntary silent pauses or blocks in which the stutterer is unable to produce sounds. 'Verbal non-fluency' is the accepted umbrella term for such speech impediments. The term stuttering is most commonly associated with involuntary sound repetition, but it also encompasses the abnormal hesitation or pausing before speech, referred to by stutterers as blocks, and the prolongation of certain sounds, usually vowels and semi-vowels. The term "stuttering", as popularly used, covers a wide spectrum of severity: it may encompass individuals with barely perceptible impediments, for whom the disorder is largely cosmetic, as well as others with extremely severe symptoms, for whom the problem can effectively prevent most oral communication. The impact of stuttering on a person's functioning and emotional state can be severe. Much of this goes unnoticed by the listener, and may include fears of having to enunciate specific vowels or consonants, fears of being caught stuttering in social situations, self-imposed isolation, anxiety, stress, shame, or a feeling of "loss of control" during speech.

Stuttering is generally not a problem with the physical production of speech sounds or putting thoughts into words. Despite popular perceptions to the contrary, stuttering does not affect and has no bearing on intelligence. Apart from their speech impediment, people who stutter may well be 'normal' in the clinical sense of the term. Anxiety, low self-esteem, nervousness, and stress therefore do not cause stuttering per se, although they are very often the result of living with a highly stigmatized disability and, in turn, exacerbate the problem.

The disorder is also variable, which means that in certain situations, such as talking on the telephone, the stuttering might be more severe or less, depending on the anxiety level connected with that activity. Although the exact etiology of stuttering is unknown, both genetics and neurophysiology are thought to contribute. There are many treatments and speech therapy techniques available that may help increase fluency in some stutterers; however, there is essentially no "cure" for the disorder at present.