The authors describe protocols for culture conditions in which mouse ES cells can be maintained in an undifferentiated state or committed to undergo adipocyte differentiation at a high rate and in a highly reproducible fashion. There is also a protocol for maintaining and differentiating human adult stem cells, isolated form adipose tissue and from bone marrow, into adipocytes. These culture systems provide a powerful means for studying the first step of adipose cell development and a means to investigate effects of drugs on the biology of adipocytes. There are also protocols for detection of adipocytes and analysis of their gene expression.