It is well established that estrogen is essential for statural growth during puberty, but until recently it was generally accepted that the role of estrogen was negligible during puberty in boys. Clinical findings in three male patients, one with an inactivating mutation in the estrogen receptor and two with P-450 aromatase deficiency, have, however, advanced our knowledge of the role of estrogen in the male. It has become apparent that estrogen (1) initiates the pubertal growth spurt, (2) causes growth plate fusion at the end of puberty, and (3) augments accrual of bone during puberty. Two estrogen receptors (alpha and beta) mediate the actions of estrogen, and the presence of both has been demonstrated in the growth plate. The mechanism by which estrogen acts locally on the growth plate, however, remains largely unknown. With the recent development of knockout models for both estrogen receptors, attempts are being made to unravel the local actions of estrogen in order to characterize further their roles in the regulation of longitudinal growth.