The true prevalence of sexual sadism (and its variants) is unknown. However, all clinicians will knowingly or unknowingly encounter patients with this disorder. Regretfully, few programs offer adequate education in normal sexuality and even less provide training in the assessment and treatment of pathologic sexual interests. This review synthesizes current theories about possible etiologies of criminal sexual sadism and the resulting implications for diagnosis and treatment of this sexual disorder. Included is a review of theories of criminally sadistic sexual motivations, response patterns, and physiology, including possible neurophysiologic factors and more complex interactions. This review focuses primarily on published English-language scientific studies of sexual sadism. It should be noted that my use of the term sadism refers to nonconsensual sexual aggression.