Psychopathy is a concept that carries strong connotations of evilness and dangerousness. In media and popular science, the psychopath is commonly portrayed either as a brutal mass murderer or as a vicious boss. Descriptions of individuals with clear psychopathic traits can be found in ancient literature, and clinical descriptions of psychopathic inpatients trace back to the 18th century. Psychopathy is an important personality construct in the forensic field, and psychopathy assessments are extensively used to inform legal decisions in North America and throughout Europe. This entry describes the leading definitions and assessments of psychopathic traits, in both criminal and noncriminal settings. Psychopathic traits in childhood and adolescence are discussed with reference to causal theories. Also included are sections on the use of psychopathy assessments for legal decision-making purposes and on past and recent approaches to treatment of psychopathic offenders.