Brown adipose tissue (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). Brown adipose tissue is found in almost all mammals. Classification of brown fat refers to two distinct cell populations with similar functions. The first shares a common embryological origin with muscle cells, found in larger "classic" deposits. The second develops from white adipocytes that are stimulated by the sympathetic nervous system. These adipocytes are found interspersed in white adipose tissue and are also named 'beige' or 'brite' (for "brown in white"). Brown adipose tissue is especially abundant in newborns and in hibernating mammals. It is also present and metabolically active in adult humans, but its prevalence decreases as humans age. Its primary function is thermoregulation. In addition to heat produced by shivering muscle, brown adipose tissue produces heat by non-shivering thermogenesis. The therapeutic targeting of brown fat for the treatment of human obesity is an active research field. In contrast to white adipocytes, which contain a single lipid droplet, brown adipocytes contain numerous smaller droplets and a much higher number of (iron-containing) mitochondria, which gives the tissue its color. Brown fat also contains more capillaries than white fat. These supply the tissue with oxygen and nutrients and distribute the produced heat throughout the body. Location and classification The presence of brown adipose tissue in adult humans was discovered in 2003 during FDG-PET scans to detect metastatic cancers. Using these scans and data from human autopsies, several brown adipose tissue deposits have been identified. In infants, brown adipose tissue deposits include, but are not limited to: interscapular, supraclavicular, suprarenal, pericardial, para-aortic and around the pancreas, kidney and trachea. These deposits gradually get more white fat-like during adulthood. In adults, the deposits that are most often detected in FDG-PET scans are the supraclavicular, paravertebral, mediastinal, para-aortic and suprarenal ones. It remains to be determined whether these deposits are 'classical' brown adipose tissue or beige/brite fat. Brown fat in humans in the scientific and popular literature refers to two cell populations defined by both anatomical location and cellular morphology. Both share the presence of small lipid droplets and numerous iron-rich mitochondria, giving the brown appearance. "Classical" brown fat is found in highly vascularized deposits in somewhat consistent anatomical locations, such as between the shoulder blades, surrounding the kidneys, the neck, and supraclavicular area, and along the spinal cord. This is the smaller of