Adipose tissue allows for triacylglycerol storage in periods of energy excess and the subsequent use of triacylglycerol stores during energy deprivation with the lipogenic/lipolytic balance under tight hormonal control. Information is incomplete on the origins of adipose cells and adipose tissue. White adipose tissue is not detected during embryonic life or at birth in rats and mice, but is present at birth in humans and pigs. Adipocytes represent between one-third and two-thirds of cells in adipose tissue. The remaining cells comprise the stromal vascular fraction and include blood cells and endothelial cells, along with adipocyte precursor cells [1]. Studies to date suggest the adipocyte lineage derives from an embryonic stem cell precursor with the capacity to differentiate to mesodermal cell types such as adipocytes, chondrocytes and myocytes. Treatment of murine C3HIOTI/2 mesodermal stem cells with a demethylating agent generates loci of muscle, cartilage and fat cells, and suggests that relatively few genes may regulate adipocyte differentiation [2, 3]. Animal studies demonstrate that thepotential to make new fat cells continues throughout the lifespan. Rats increase fat-cell number in response to high-carbohydrate or high-fat diets. Fat depots of very old mice contain cells that express early differentiation markers, suggesting an ongoing process ofdifferentiation. The cells of the stromal vascular fraction of adipose tissue from aged rats and humans differentiate in vitro into adiposecells [4, 5]. This review addresses the regulation of adipose development. Since this area has been the subject of intensive research, we focus here on conclusions attained through the use of in vitro model systems of adipocyte differentiation. The first section describes the various preadipocyte cell lines and reviews the changes in gene expression that occur during their differentiation. Next, the culture conditions requisite for, and the hormonal regulation of, adipocyte differentiation are reviewed and the involvement of cytoskeletal and extracellular matrix is examined. This is followed by an overview of cis-elements and DNA-binding proteins that are involved in the expression of adipocyte genes. Lastly, we present the approaches taken to attempt to identify genes that may control not just the expression of specific adipocyte genes, but the adipocytedifferentiation programme.