Nuclear receptors (NRs) are major targets for drug discovery and play key roles in development and homeostasis, as well as in many diseases such as obesity, diabetes and cancer. This review provides a general overview of the mechanism of action of nuclear receptors and explores the various factors that are instrumental in modulating their pharmacology. One of the most promising aspects of nuclear receptor pharmacology is that it is now possible to develop ligands with a large spectrum of full, partial or inverse agonist or antagonist activities, but also compounds, called selective nuclear receptor modulators, that activate only a subset of the functions induced by the cognate ligand or that act in a cell-type-selective manner.