Adipocytes, which vary enormously in size (20- 200 µm in diameter), are embedded in a connective tissue matrix and are uniquely adapted to store and release energy. There are two types of adipose tissue: white adipose tissue (WAT) and brown adipose tissue (BAT). These two tissues are made up of two types of fat cells. WAT is the major energy reserve in higher eukaryotes, storing triacylglycerol in periods of energyexcess and mobilizing during energy deprivation. Until recently, the adipocyte has been considered only a passive tissue for the storageof excess energy in the form of fat. However, there is now compelling evidence that adipocytes act as endocrine secretory cells. It has been shown that several hormones, growth factors, and cytokines are actually expressed in white adipose tissue. Through a network of local and systemic signals, which interact with neuroendocrine regulators, adipose tissue signaling pathways, arranged in a hierarchical fashion, constitute one of the voices of the body that enable the organism to adapt to a range of different metabolic challenges such asstarvation, stress, and infection, as well as periods of gross energy excess. At Cell Applications, Inc., primary cultures of adipocytes are prepared by inducing differentiation of human primary preadipocytes and are pre-screened for expression of these important adipocyte functional proteins.