Gender has become one of the most reliable predictors of crime, especially violent crime. Governmental, crime victim, public, private, and self-reporting data sources are consistent in indicating adult males commit more crimes and are more likely to repeat their crimes than their adult female counterparts. Even with juvenile data denoting marked increases in girls committing more serious violent crimes, the overwhelming prevalence of male’s criminal involvement continues. This entry begins by reviewing statistical data regarding crime and gender and then examines theories related to criminality among genders as well as the role that genes may play in criminality. The entry concludes with a brief look at gender in the criminal justice system.