The ovarian follicle is the fundamental unit of the ovary. It contains the oocyte that may eventually ovulate, undergo fertilization and form an embryo. It also provides the steroid and protein hormones required for maintenance of the ovarian cycle, the secondary sex characteristics and preparation of the uterus for implantation. Follicle formation and folliculogenesis have been well documented for many mammalian species. However, the control of follicular reserves and entry of follicles into the growth path towards atresia or ovulation are not well understood. We have investigated several aspects of follicle formation and folliculogenesis by (a) using unbiased assumption-free stereological methods to accurately count follicles, particularly primordial follicles in the follicular reserve, (b) testing the effects of members of the transforming growth factor-β family on folliculogenesis and follicle function, and (c) examining the role of estrogen in folliculogenesis using the aromatase knock-out mouse model. These studies are summarized and reviewed