Tooth clenching or grinding may cause serious problems for patients and dentists. Symptoms occur at the level of the teeth (wear), the periodontium and the musculoskeletal system. In the past, the definition of bruxism often was vague and controversial, and also a clinical diagnosis appeared to be difficult. More recently research in the sleep laboratory, using a combination of electromyography video- and sound recording, made it possible to distinguish between bruxism and other oral behaviour. The association between pain and dysfunction of the masticatory system with bruxism is still unclear.