According to data from the Federal Bureau of Investigation, from 1991 to 2000, arrests of girls increased more (or decreased less) than arrests of boys for most types of offenses. By 2004, girls accounted for 30 percent of all juvenile arrests. However, questions remain about whether these trends reflect an actual increase in girls' delinquency or changes in societal responses to girls' behavior. To find answers to these questions, the Office of Juvenile Justice and Delinquency Prevention convened the Girls Study Group to establish a theoretical and empirical foundation to guide the development, testing, and dissemination of strategies to reduce or prevent girls' involvement in delinquency and violence. The Girls Study Group Series, of which this bulletin is a part, presents the Group's findings. The series examines issues such as patterns of offending among adolescents and how they differ for girls and boys; risk and protective factors associated with delinquency, including gender differences; and the causes and correlates of girls' delinquency.