There are no gender differences in depression rates in prepubescent children, but, after the age of 15, girls and women are about twice as likely to be depressed as boys and men. In this article, three models for how gender differences in depression might develop in early adolescence are described and evaluated. According to Model 1, the causes of depression are the same for girls and boys, but these causes become more prevalent in girls than in boys in early adolescence. According to Model 2, there are different causes of depression in girls and boys, and the causes of girls' depression become more prevalent than the causes of boys' depression in early adolescence. According to Model 3, girls are more likely than boys to carry risk factors for depression even before early adolescence, but these risk factors lead to depression only in the face of challenges that increase in prevalence in early adolescence. Most studies of gender differences in depression have focused on the effects of individual variables on depression in girls and boys rather than on testing models of how these differences develop. Evidence for the variables most commonly thought to contribute to gender differences in depression in children and adolescents is reviewed, and this evidence is related to the three models for how these differences develop. It is concluded that Model 3 is best supported by the available data, although much more research is needed. Before adolescence, girls appear to develop more risk factors for depression than boys; girls also apparently face more new challenges in early adolescence than boys. It is argued that these factors combine, as specified in Model 3, to generate gender differences in depression beginning in early adolescence.