Abnormal psychology can broadly be defined as the application of the principles of psychology to the study of mental disorders, including research into the causes and treatment of psychopathologies. The controversial question of major consideration in this field centers on what constitutes abnormal behavior. The DSM-IV requires that behavior meet three general diagnostic inclusion criteria in order to be considered as clinically significant and, therefore, qualify as a mental disorder: present distress, disability, and significantly increased risk of suffering death, pain, or an important loss of freedom. In order to be classified as such, then, a mental disorder must confer great personal distress, impair physical or mental abilities, and confer biological disadvantages upon the individual that do not represent statistical norms. The exclusion criteria basically states that if behavior is an expectable and culturally sanctioned reaction to a particular event, deviant in form, such as the action of political or religious minorities, or a conflict between the individual and society, it cannot be considered as a mental disorder. This criteria, thus, is attempting to decrease cultural biases and control for any environmental event that could serve as a confounding variable.

As with any system, however, there are limitations within the definitions and characteristics outlined by the inclusion and exclusion criteria. For instance, some disorders, such as antisocial personality disorder do not cause the individual to feel personal distress. Furthermore, some things that are rare or do not represent statistical norms, such as geniuses, can be seen as good, not to mention that things such as a fear of heights can be biologically advantageous in that it actually reduces the risk of dying or suffering.

Some of the most prominent forms of psychopathology are schizophrenia and other psychotic disorders, mood disorders, anxiety disorders, personality disorders, dissociative and somatoform disorders. Other important disorders, which have had major recent research advancements into their causes and effective treatment, include substance-abuse/dependence disorders, childhood disorders, eating disorders, and sexual and gender identity disorders.