Much of the research devoted to understanding adipose tissue development is currently performed in vitro. Several cell culture models, including preadipocyte cell lines and primary culture of adipose-derived stromal vascular precursor cells, are commonly used to study molecular and cellular events and regulatory influences on preadipocyte proliferation and differentiation. Primary preadipocyte culture systems have several distinct advantages over preadipose cell lines. Because they have not been passaged continuously in culture, primary cultures of adipose derived stromal-vascular (SV) cells more closely reflect the in vivo characteristics of the tissue from which they are derived. In addition, primary cells can be obtained from various adipose tissue depots and from animals at different stages of development, from early postnatal life through advanced age. Cells can also be obtained from genetic rodent models of obesity or from rats and/or mice subjected to nutritional or hormonal manipulation. In each case, specific adipose tissue depots are dissected and the SV cells obtained after collagenase digestion. To examine the effect of tissue source or in vivo or in vitro treatment on preadipocyte proliferation, SV cells are labeled by thymidine incorporation during the exponential growth phase and maintained in culture until sufficiently lipid-filled to allow separation by density. Regulatory influences on various stages of preadipocyte differentiation can be examined in rat SV cultures in a controlled environment featuring chemically defined serum-free medium; whereas, more temperamental mouse SV cultures require the presence of serum for optimal differentiation. Alternatively, preadipocytes differentiated in vitro may be used for examining adipocyte metabolic or secretory responses.