Sexual abuse of children by other children or adolescents constitutes approximately 40 to 90 percent of sexual offending against children in Australia. Intrafamilial adolescent sex offenders (IASOs) have a mixed profile, but differ in significant ways from adult incest offenders. IASOs apparently have greater developmental trauma and family dysfunction than adult incest offenders. The current study confirmed previous studies that have shown IASOs who sexually offended against siblings had a history of sexual abuse. IASOs also manifest more serious behavioral difficulties related to conduct disorders compared with extrafamilial adolescent and adult sex offenders. The most common disorders are attention-deficit/hyperactivity disorder and posttraumatic stress disorder. Treatment programs for adolescent sex offenders are based primarily on those developed for adult sex offenders. Developmental issues and family influences are not given the priority that is needed for these young sex offenders. Most treatment programs use a combination of group and individual therapies, with family therapy being a feature of only a few programs. Cognitive behavioral therapy (CBT) is the most common model of therapeutic intervention with adolescent sex offenders. It is widely used in Australia and internationally. CBT targets deviant sexual arousal, poor sexual impulse control, deficits in empathy, social skills, relapse prevention, and cognitive distortions. Treatment programs that combine a variety of treatment modalities produce more promising outcomes, however. In the course of treating adolescents' inappropriate and harmful sexual behaviors, their behaviors should be viewed as a health issue rather than a moral or criminal issue. This study used the findings of a 4-year study in Western Australia that collected data on 38 IASOs before, during, and after treatment. 2 tables 1 figure, and 56 references