Brown fat is a type of adipose tissue present in many newborn or hibernating mammals. In contrast to white adipocytes (fat cells), which contain a single, large fat vacuole, brown adipocytes contain several smaller vacuoles and a much higher number of mitochondria. Brown fat also contains more capillaries (e.g. for oxygen supply).

The mitochondria in a eukaryotic cell utilize fuels to produce energy (in the form of ATP). Mitochondria, first pump protons against a gradient (from low to high concentration), and then let them "fall" back, releasing their energy to form ATP. In endothermic animals, body heat is maintained by signalling the mitochondria to let protons run back along the gradient without producing ATP. The 'wasted' energy is released as heat. While this happens in all cells of endotherms to some amount, especially when body temperature is below a regulatory threshold, brown fat tissue is highly specialized for this non-shivering thermogenesis.

In neonates (new born babies), brown fat, which then makes up about 5% of the body mass and is located on the back, along the upper half of the spine and towards the shoulders, is of great importance to avoid lethal cold (hypothermia is a major death risk for premature neonates). Numerous factors make infants more susceptible to cold than adults:

• the higher ratio of body surface (proportional to heat loss) to body volume (proportional to heat production)
• the higher proportional surface area of the head (which is seldom cloth-covered)
• the low amount of musculature and the inability or reluctance to shiver
• a lack of thermal insulation, e.g. subcutaneous fat and fine body hair (especially in prematurely born children)
• the inability to move away from cold areas, air currents or heat-draining materials
• the inability to use additional ways of keeping warm (e.g. turning up a heating, drying their skin, changing clothes or performing physical exercise)
• the nervous system is not fully developed and does not respond quickly and/or properly to cold (e.g. by contracting blood vessels in the skin)

When growing up, most of the mitochondria (which are responsible for the brown color) in brown fat disappear, and the tissue becomes similar in function and appearance to white fat, as a mere fat deposit, though some adults do retain their brown fat.