Abnormal psychology is the branch of psychology that studies unusual patterns of behavior, emotion and thought, which may or may not be understood as precipitating a mental disorder. Although many behaviors could be considered as abnormal, this branch of psychology typically deals with behavior in a clinical context. There is a long history of attempts to understand and control behavior deemed to be aberrant or deviant (statistically, functionally, morally or in some other sense), and there is often cultural variation in the approach taken. The field of abnormal psychology identifies multiple causes for different conditions, employing diverse theories from the general field of psychology and elsewhere, and much still hinges on what exactly is meant by "abnormal". There has traditionally been a divide between psychological and biological explanations, reflecting a philosophical dualism in regard to the mind-body problem. There have also been different approaches in trying to classify mental disorders. Abnormal includes three different categories; they are subnormal, supernormal and paranormal.

The science of abnormal psychology studies two types of behaviors: adaptive and maladaptive behaviors.