The recent discovery of functional brown adipose tissue (BAT) in adults has stimulated intense interest for its therapeutic potential against obesity and its related metabolic conditions. Although rodent studies support the importance of brown and beige adipocytes in metabolic health, a smaller number of clinical investigations have examined their physiological significance in people. This chapter provides a critical overview of the published research regarding the metabolic role of BAT and the beiging of white adipose tissue (WAT) in people. In summary, the current available evidence supports the physiological relevance of human BAT in energy, glucose and lipid metabolism. The ability of human WAT to undergo beiging has only recently been demonstrated in patients with severe burn trauma, while its metabolic role remains currently unknown. Future research is needed to gain deeper understanding of the regulating mechanisms and metabolic role of brown and beige adipocytes in health and disease.