In this review, it is assumed that the reader is familiar with the basic properties of white and brown adipose tissue. The role and metabolic characteristics of both tissues in adult mammals have been adequately and repeatedly reviewed (1-3) and will be mentioned here only insofar as they impinge upon developmental aspects. Macroscopically brown fat is found in particular sites of the body, usually under the skin between the shoulder blades, in the axilla, and surrounding the aorta and the kidneys. It is assumed to have a brown appearance because of its high cytochrome c content. A typical brown adipose tissue cell contains many densely packed mitochondria and several smaller fat vacuoles. A white fat cell during fetal and perinatal development may appear to be very similar under the microscope, and even today it has not been settled definitely whether or not brown fat changes to white fat during ontogeny or whether they are separate tissues. The development (ontogenic) of white adipose tissue is fairly well known with regard to structure, cell size, and some microscopic changes. However, metabolic development has been inadequately described and studied. This is particularly true for mammals other than man. In man, it is now possible to describe in some detail early postnatal changes and to compare them with adult parameters. The opposite is true of brown adipose tissue. This tissue has been examined fairly thoroughly in rats, guinea pigs, and rabbits. However, ontogenetic changes in man are unknown. This is due to the fact that it is relatively easy to obtain samples of white fat from normal newborns but almost impossible to sample brown fat from the same source. first discuss the development of brown fat. This has been reviewed several times (4, 5). Data contained in these reviews will only be touched upon.