For many years, it was known that small animals and human infants expressed significant amounts of brown fat, but there was scant evidence for it playing a significant role in adult humans. In the newborn human, thermogenesis by brown-fat mitochondria is vital to survival, as it is in hibernating mammals. In fur seals and other animals naturally acclimated to the cold, muscle-cell mitochondria contain thermogenin; as a result, much of the proton-motive force is used for generating heat, thereby maintaining body temperature. Recently investigators have used sophisticated functional imaging methods (such as positron-emission tomography) to definitively establish the presence of brown fat in adult humans in the neck, clavicle, and other sites, the levels of which are significantly increased upon exposure to cold. Furthermore, detailed analyses of the biochemical properties and developmental origins of thermogenic fat cells have uncovered the existence of at least two subtypes of such cells: classic brown-fat cells that develop from precursor cells also used to generate skeletal muscle cells, and beige-fat cells whose detailed properties (gene expression pattern, responses to hormonal signals) differ from those of brown-fat cells. Further characterization of beige-fat cells and their influence on normal metabolism and disease may lead to new approaches to treat or prevent some metabolic disorders.