Since animals are under constant threat of starvation, storage of energy sources inside the body is essential for various activities. Therefore, animals exhibit highly sophisticated mechanisms for storing energy inside their bodies in adipose tissue. However, in humans it has been clarified that fat cell (adipocyte, which comprises adipose tissues) dif-ferentiation and the extent of subsequent fat accumulation (hypertrophy of cells) are closely associated with the occurrence and advancement of various diseases resulting from obesity. Moreover, progress in biochemical studies with respect to adipocyte in recent years has rap-idly clarified new functions and differentiation mechanisms of adipocytes. Interesting points, in particular, are the function of white adipocytes as “secreting cells” and the molec-ular mechanism of adipocyte differentiation via the nuclear receptors. Consequently, adi-pose tissue is being targeted to prevent or treat many common diseases. This paper summa-rizes recent knowledge on characteristics, differentiation and proliferation of adipocytes and the mechanisms by which adipocytes are regulated.