Two types of adipose tissues, brown and white, coexist in mammals. WAT stores energy in the form of triglycerides while BAT has the ability to dissipate energy through adaptive thermogenesis and has a common embryological origin with skeletal muscle. A particular type of adipocytes sometimes occurs within classical WAT depots. These adipocytes manifest several classical brown adipocyte characteristics, most notably the presence of UCP1, but do not express the novel molecular markers characteristic of classical brown adipocytes and have completely independent developmental origin. This particular adipose cell-type cannot be classified either as brown or white; it represents a distinct – “brite” (brown like-in-white) – adipose cell-type (sometimes referred as beige, inducible or convertible adipocytes). Subcutaneous white-fat depots (i.e. inguinal depot) are particularly prone to britening under prolonged cold exposure. In mitochondria isolated from the inguinal “white” adipose depot of cold-acclimated mice, UCP1 protein levels almost reach those in brown-fat mitochondria. The UCP1 is thermogenically functional – these mitochondria exhibit UCP1-dependent thermogenesis with lipid or carbohydrate substrates with canonical guanosine diphosphate (GDP) sensitivity and loss of thermogenesis in UCP1 knockout (KO) mice. The thermogenic density (UCP1-dependent oxygen consumption per g tissue) of inguinal white adipose tissue is maximally one-fifth of interscapular brown adipose tissue, and the total quantitative contribution of all inguinal mitochondria is maximally one-third of all interscapular brown-fat mitochondria. Thus, the classical brown adipose tissue depots would still predominate in thermogenesis.