An overview of glucocorticoid action is presented including aspects of historic, clinical, and physiologic interest. Despite the diversity of effects, glucocorticoid hormone action at the cellular level simplifies to a rather universal model that involves binding to a soluble receptor, followed by interaction of the complex with the chromatin and modification of gene expression. This unitary concept has important implications in pathology, pharmacology, and therapeutics. Finally, a definition of a glucocorticoid in terms of its receptor is presented, which we feel is better than the traditional terminology based on specific biological effects. It is hoped that this review will also provide the reader with a framework in which the various contributions in the Monograph can be integrated.