E strogen receptors, PPARs, and liver X receptors are members of the nuclear receptor superfamily of ligand-dependent transcription factors that regulate diverse aspects of development and homeostasis. Recent studies of the biologic roles of these receptors and their mechanisms of action have significantly advanced our understanding of transcriptional programs that control lipid and carbohydrate metabolism, immunity and inflammation, and wound repair. These findings provide insights into the therapeutic actions of existing drugs that target nuclear receptors and raise new possibilities for development of safer, more effective drugs for the prevention and treatment of metabolic and cardiovascular diseases. In this introduction to this Review series, underlying mechanisms that enable nuclear receptors to positively and negatively regulate gene expression are presented as background to the focused reviews on estrogen receptors, PPARs, liver X receptors, and the PPARγ coactivator-1 (PGC-1) family of coactivators.