Adipose tissue is not just an organ for storage and release of energy rich lipids. It has also an important signalling function. The fatty acids released through lipolysis of triglycerides within the fat cells act as regulators of several hormonal functions in other organs and they serve also as transcriptional regulators. Moreover adipose tissue secretes a number of proteins (termed adipokines). They function as autocrine/paracrine factors within the fat tissue but are also released into the circulation and act as endocrine factors in other organs. Two fat cell specific hormones are described, namely leptin which regulates appetite and energy expenditure and adiponectin which acts as an "insulin sensitizer". Several metabolic disorders are accompanied by changes in the adipose tissue signalling, in particular obesity. A key factor behind these disturbances in adipose tissue is a local low-grade inflammation. This, in turn, may cause an increased production of cytokines and chemokines within adipose tissue with consequences for the metabolic and endocrine function of this organ. Not all obese have metabolic complications and some lean and apparently healthy ones have metabolic abnormalities which can be linked to a malfunction of adipose tissue. The mechanisms and roles of this plasticity of adipose tissue function will be discussed with focus on the findings in humans.