In early postnatal development, growth of adipose tissue is due to both cellular hypertrophy and hyperplasia. Adipose cell (adipocyte) hypertrophy is the major mechanism in fattening of ruminants grown to market weight, although evidence is accumulating that preadipose cells can proliferate postnatally, even in mature animals. In interfasicular adipose tissue (marbling), however, small adipose cells are present and their number makes a positive contribution to the size of this fat depot in ruminants of market weight. Present information does not indicate whether these small cells are newly synthesized cells or are cells that differentiated early in postnatal development and fill with lipid at some later time. Limitations on detecting small adipose cells in cell-counting techniques are partly responsible for conflicting conclusions on the cellular basis for adiposity. Nutritional modification of adipose cell number has been reported in rodents. However, the extreme nutritional modifications required to alter cell number have little practical application in the growth of ruminants for meat production. Adipose cells of various sizes respond differently in the esterification and synthesis of fatty acids. The greater rates of lipid synthesis from acetate in large adipose cells may be related to increased uptake of substrate in cells with a large surface area.