In mammals, the adipose organ is a multi-depot organ made of two tissue types, the white and brown adipose tissues, which collaborate in partitioning the energy contained in lipids between thermogenesis and the other metabolic functions. It consists of several sc and visceral depots. Some areas of these depots are brown and correspond to brown adipose tissue, while many are white and correspond to white adipose tissue. White areas contain a variable amount of brown adipocytes and their number varies with age, strain and environmental conditions. Brown and white adipocyte are morphologically different. At light microscopy level, brown adipocytes have cytoplasmic lipids arranged as numerous small droplets (multilocularity), while white adipocytes have cytoplasmic lipids arranged in a unique vacuole (unilocularity). Ultrastructurally, brown adipocytes have numerous big mitochondria packed with cristae and containing the thermogenic uncoupling protein 1 (UCP1). In vivo and in vitro studies have shown that the differentiation process of brown and white adipocytes shows distinctive features. Nevertheless, the origin of the adipocyte precursor is still unknown. Recent data have stressed the plasticity of the adipose organ in adult animals. Indeed, under peculiar conditions fully differentiated, white adipocytes can transdifferentiate into brown adipocytes, and viceversa. The ability of the adipose organ to interconvert its main cytotypes in order to meet changing metabolic needs is highly pertinent to the physiopathology of obesity and related to therapeutic strategies.