Unipolar depression only becomes more common in girls than boys after the age of 13, as a result of an increased incidence of depressive episodes in girls at that time. This article reviews evidence that links multiple dimensions of maturation in the hypothalamo-pituitary-gonadal axis with this phenomenon. Effects of developmental status and timing have been implicated, but few studies have deployed either the measurement strategies or the statistical power needed to provide a satisfactory answer to the question regarding which components of puberty are most responsible.