Endocrinology of adipose tissue has become an exciting research field in which revolutionary new findings can be expected. Especially since the discovery of the adipocyte-derived hormone leptin, the traditional view of adipose tissue has rapidly changed. This review summarizes current knowledge about adipose tissue formation during the embryonal phase and its further development in childhood and adolescence. Classical functions of adipose tissue, e.g., storage and mobilization of energy, and its recently discovered function as an endocrine organ will be explained. In addition, the involvement of adipocyte-derived factors in the development of obesity-associated comorbidities will be discussed in detail.