Growth and differentiation of the mammalian oocyte is regulated with the coordinate development of the granulosa cells. The complex signaling pathways that regulate the growth and development of mammalian oocytes are beginning to be elucidated through the use of gene targeting. These technologies have provided new insight into the roles of specific genes during the development of the germ cells and gonads, as well as post-pubertal development of oocytes. In many cases, these studies have resulted in a new understanding of the function of certain genes, in others they have provided new genes and pathways to be studied in mammalian reproductive biology. Ultimately, these studies will shed light on human genetic disease and infertility.