The transition into puberty is a critical developmental period associated with profound biological and psychosocial changes. During early adolescence there is a dramatic rise in depressive symptoms in girls compared with boys. By the mid-teens, girls are twice as likely to experience depression than boys, and this gender difference persists through adult life. Adolescence is, therefore, the key developmental period for the female rise in depression. The timing of the pubertal transition is of interest because early puberty in girls has been linked to a range of adverse outcomes in psychological, social, sexual and educational domains. Girls who mature earlier than their peers face a cascade of psychosocial challenges that may be inconsistent with their level of cognitive and emotional development. This may lead to feelings of isolation and increased vulnerability to depression. It is still unclear, however, whether adverse outcomes associated with early puberty translate into longer-term negative consequences for mental health in young women. Timing of puberty could also be important in determining risk for depressive symptoms/depression in boys, but findings are inconsistent and more research is needed. Few studies have examined mechanisms through which timing of puberty affects risk for depressive symptoms/depression. Psychosocial theories propose that individual and social factors explain more of the variance in adolescent depression than hormones alone.