It is widely accepted that newborn humans are provided with brown adipose tissue (BAT) and that adult humans lack, or have only a small amount, of it. Therefore the physiological role of BAT in humans is debated. It is quite clear that BAT in rodents has an important role in the prevention and therapy of obesity and diabetes and specific drugs can induce BAT development in adult animals. New concepts regarding the biology of adipose tissues in mammals have been developed during the last years leading to the hope for the development of BAT in human adults as a new challenge for the treatment of obesity and related diseases. These new concepts are basic to understanding the above-proposed therapeutic strategy and are the concept of the adipose organ and the concept of transdifferentiation. In this paper these new concepts will be explained together with a review of available scientific data on human BAT.