The study of interleukin-23 (IL-23) over the past 8 years has led to the realization that cellular immunity is far more complex than previously appreciated, because it is controlled by additional newly identified players. From the analysis of seemingly straightforward cytokine regulation of autoimmune diseases, many limitations of the established paradigms emerged that required reevaluation of the 'rules' that govern the initiation and maintenance of immune responses. This information led to a major revision of the T-helper 1 (Th1)/Th2 hypothesis and discovery of an unexpected link between transforming growth factor-beta-dependent Th17 and inducible regulatory T cells. The aim of this review is to explore the multiple characteristics of IL-23 with respect to its 'id' in autoimmunity, 'ego' in T-cell help, and 'superego' in defense against mucosal pathogens.