In the ovary, Anti-Müllerian hormone (AMH) is produced by the granulosa cells of early developing follicles and inhibits the transition from the primordial to the primary follicular stage. AMH levels can be measured in serum and have been shown to be proportional to the number of small antral follicles. In women serum AMH levels decrease with age and are undetectable in the post-menopausal period. In patients with premature ovarian failure AMH is undetectable or greatly reduced depending of the number of antral follicles in the ovaries. In contrast, AMH levels have been shown to be increased in women with polycystic ovary syndrome (PCOS). AMH levels appear to represent the quantity of the ovarian follicle pool and may become a useful marker of ovarian reserve. AMH measurement could also be useful in the prediction of the extremes of ovarian response to gonadotrophin stimulation for in vitro fertilization, namely poor- and hyper-response. Although AMH has the potential to increase our understanding of ovarian pathophysiology, and to guide clinical management in a broad range of conditions, a number of important questions relating to both the basic physiology of AMH and its clinical implications need to be answered.