Maturation of adipose tissue results from both the expansion of mature adipocytes and the formation of new adipocytes from adipocyte precursor cells. A variety of hormones related to adipogenesis have been identified recently. Both growth hormone (GH) and insulin-like growth factor I (IGF-I) are of major significance in adipocyte differentiation. IGF-I has been suggested to be a major regulator of cell proliferation, differentiation and metabolism, thus regulating, among other biological processes, adipose tissue growth and differentiation of pre-adipocytes into adipocytes. GH exerts its effects by increasing the pool of adipocyte precursor cells capable of differentiating into mature adipocytes. In addition, GH seems to have the potential to reduce the volume of mature adipocytes, thus inhibiting the expansion of adipose tissue and reducing body fat. This chapter gives an overview of studies that have investigated the roles of insulin, GH and IGF-I in adipogenesis.