Although a thermogenic role for brown adipsoe tissue (BAT) is now well accepted for adult rodents and mammalian neonates exposed to cold, such a role has not been considered relevant for adult man, as adult man has been believed not to possess significant amounts of BAT. In recent tears, it has however become evident from PET-scanning studies that adult man in general possesses BAT, which is cold-activated and regulated by the sympathetic nervous system, just as in rodents. The sympathetic nervous system regulates not only the acute activity of the tissue but also its growth and development in the cold. We have recently shown that one limitation in the growth of the tissue seems to be angiogenesis, since removal of a modulatory signal allowing greater angiogenesis, also resulted in greater thermogenic capacity. One unexpected new finding is that brown adipocytes develop from the same lineage as skeletal muscle cells and not from a common lineage with white adipocytes, as had been previously anticipated. While a role for BAT thermogenesis in body weight maintenance was proposed many years ago, it has only recently been unequivocally demonstrated that indeed the absence of BAT activity in mice does result ni obesity, even on a non-obesogenic diet. This result was found only when housing mice at thermoneutral temperatures i.e. by "humanizing" the mice. In consequence, it is now possible to consider development and activation of BAT as a potential therapy for human obesity.