The role of androgens in development of male reproductive organs is well documented. The role of estrogens in the development of male reproductive organs remains largely unknown; although both estrogen receptors and aromatase enzyme have been identified in the developing penis of a number of species, including humans. Male offspring of women exposed to diethylstilbestrol during pregnancy have higher incidences of epididymal cysts, cryptorchidism, hypospadiasis, and smaller testes. Since female hormones were routinely prescribed to treat threatened pregnancy and considering the potential implications of female hormones during prenatal period on the development of male reproductive system, the present book describes the effect of prenatal exposure to progesterone on adult male reproduction. Significant deterioration in reproduction was observed in mice exposed to progesterone during embryonic development which includes reduction in steroidogenesis and spermatogenesis. Testosterone supplementation during post-natal period partially restored the suppressed reproduction. This book provides valuable information to those working in mammalian reproduction and also to the interested layman.