Abnormal psychology is the study of abnormal behavior in order to describe, predict, explain, and change abnormal patterns of functioning. Abnormal psychology studies the nature of psychopathology and its causes, and this knowledge is applied in clinical psychology to treat a patient with psychological disorders.

In the study of abnormal behavior, it can be difficult to define the line between which behaviors are considered normal and which are not. In general, abnormal behaviors must be maladaptive and cause an individual subjective discomfort (signs of emotional distress). Generally, abnormal behaviors are classified as:

Abnormal as in "infrequent" in relation to the overall population.
Abnormal as in "maladaptive". The behavior fails to promote well being, growth, and fulfillment of a person.
Abnormal as in "deviant". The behavior is not socially acceptable.
Abnormal as in "unjustifiable". The behavior that cannot be rationalized.