Conduct disorder (CD) is the second most common psychiatric disorder in girls with population rates varying from 0.8% to 16% (Zoccolillo, 1993). Further, one half of the girls who have five or more conduct symptoms have a second, nonexternalizing diagnosis (e.g., major depression or schizophrenia), suggesting high comorbidity rates for girls with conduct problems (e.g., Loeber & Keenan, 1994; Robins & Price, 1991). Short- and long-term outcomes for antisocial girls include internalizing problems and mental health disorders, early pregnancy, future arrests, school drop-out, drug problems, health-risking sexual behavior, welfare dependence, and mortality (Bardone, Moffitt, & Caspi, 1997; Keenan, Loeber, & Green, 1999; Lewis, Yeager, Cobham-Portorreal, Showalter, Anthony, 1991; Pajer, 1998; Pawlby, Mills, & Quinton, 1997; Robins & Price, 1991; Zoccolillo & Rogers, 1991). Because antisocial behavior has such negative consequences for the girl and for society, effective treatment programs must be developed and implemented in adolescent populations. In this chapter, we review the theory and research on antisocial behavior in girls and present preliminary data from a randomized clinical treatment trial aimed at reducing antisocial behavior in girls in the juvenile justice system. We conclude by highlighting sexual behavior and peer relations as specific areas for future intervention work.