Adipose tissue is traditionally regarded only as an energy storage and a filler between tissues/organs. However, other important functions of adipose tissue begin to be discovered with the discovery of adipokines. Adipose tissue is also an endocrine organ, which secrets different kinds of adipokines, affecting the target organs through autocrine, paracrine and the blood circulation, and playing important roles in the pathogenesis of the obesity, insulin resistance and type 2 diabetes mellitus. This paper reviews the signal transduction pathways of adiponectin, leptin and resistin, and their changes during obesity.