This leaflet provides a summary of research and recent advances in understanding stuttering. Stuttering is defined as a disorder in which the rhythmic flow of speech is frequently disrupted by repetitions of sounds or syllables. The leaflet outlines possible causes of stuttering, methods of measurement of fluency and work on a uniform method of measurement, common factors involved with stuttering, and the process of speech formation. Relevant activities of the National Institute on Deafness and Other Communication Disorders are described, and a list of four organizations to call on for additional information is provided. (JDD)