Whereas we earlier thought about the existence of just two different adipose tissues – heat-producing brown and energy-storing white – we now see a much broader spectrum of tissues and cells; – and whereas we earlier thought that brown and white adipose tissues were closely related tissues, as the names imply, we now realize that brown adipose tissue is really developmentally a kind of muscle tissue – and although we do not really know the developmental origin of white adipose tissue, it is clearly not of muscle origin. These different developmental backgrounds are highly reflected in the character of metabolism in the tissues. To this should be added that certain cells (presently called brite or beige) within certain white adipose tissue depots may be able to express the (earlier considered) unique protein of brown adipose tissue: UCP1. The functional significances of the tissues for thermogenesis is strongly related to their ability to express sufficiently high levels of UCP1 to allow them to play a role in systemic metabolism. Whereas the issue of functional significance of the brown and white adipose tissue was earlier an issue mainly of academic interest – as it was thought that brown adipose tissue was not found in adult humans – the present understanding that brown fat exists and is active in adult humans has lifted the issue of the functional significance of the brown adipose tissue to being an enticing and possibly important issue in human metabolic research.