Epidemiological studies show that one out of four Dutch individuals suffers from a mental disorder. Consequently, dentists are confronted with noticeable deviant or problematic behaviour. In this paper an overview is presented of several of psychological problems and their relevance to dental treatment. This is illustrated by a case description. For a proper diagnosis and adequate treatment planning it is important that dentists have some general knowledge of psychological functioning of patients, and the possible psychological determinants of illness and well-being. This would prevent unnecessary damage of dental tissues and needless suffering but also frustration, both for the patient and the dentist.