Dental Sleep Medicine is the discipline concerned with the study of the oral and maxillofacial causes and consequences of sleep-related problems. Within the framework of this newly proposed definition, the discipline covers the following conditions: orofacial pain; oral moistening disorders (oral dryness, hypersalivation); gastro-esophageal reflux disorder; sleep-related breathing disorders (snoring, ob-structive sleep apnea); and mandibular movement disorders (dyskinesia, dystonia, sleep bruxism). This article briefly outlines these conditions, with particular emphasis on whether they have adverse effects on patients, and when a condition should be regarded as a disorder and therefore diagnosed and if necessary managed. This is found to be the case with virtually all these conditions, except with sleep bruxism, which does not have adverse effects on everyone, in which case it counts as ‘behavior’. Diagnosing and managing behavior is controversial, especially since sleep bruxism also appears to have positive effects. A cautious approach is therefore called for.