There has been a great deal of recent progress in our understanding of the transcriptional control of adipogenesis. Current data suggest that fat cell differentiation involves an interplay between the C/EBP family of transcription factors and PPARgamma. The thermogenic program of brown fat cells may also include a contribution from a new coactivator, PGC-1. Recent data suggests that this coactivator is responsible for activation of thermogenesis and oxidative metabolism in both brown fat and muscle. The PGC-1 dependent program includes both mitochondrial biogenesis and tissue-specific expression of uncoupling proteins.