In this paper, we study the effect of the timing of puberty on educational achievement and examine to what extent the gender differences in the timing of puberty can explain gender differences in achievement. We use British cohort data that combine information on pubertal development with test scores, behavioral outcomes as well as final educational attainment and earnings. Controlling for age 7 cognitive skills and family background, we show that late pubertal development is associated with a slower rate of cognitive skill growth during adolescence. This disadvantage in cognitive development is also reflected in lower levels of educational attainment and earnings for late developed individuals. The number of late developing boys is however too small to explain more than a fraction of the gender gap in educational outcomes. Furthermore, we find no effects on self-discipline or other behavioral outcomes in adolescence, suggesting a mechanism wholly separate from other causes of the gender gap.