Brown and beige adipocytes expend chemical energy to produce heat and are therefore important in regulating body temperature and body weight. Brown adipocytes develop in discrete and relatively homogenous depots of brown adipose tissue, whereas beige adipocytes are induced to develop in white adipose tissue in response to certain stimuli - notably, exposure to cold. Fate-mapping analyses have identified progenitor populations that give rise to brown and beige fat cells, and have revealed unanticipated cell-lineage relationships between vascular smooth muscle cells and beige adipocytes, and between skeletal muscle cells and brown fat. In addition, non-adipocyte cells in adipose tissue, including neurons, blood vessel-associated cells and immune cells, have crucial roles in regulating the differentiation and function of brown and beige fat.