Interleukin-2 receptor (IL-2R) signaling regulates tolerance and immunity. Here, we review recent work concerning the structure, signaling, and function of the IL-2R, emphasizing the contribution of IL-2 for T cell-dependent activity in vivo. IL-2R signaling influences two discrete aspects of immune responses by CD8⁺ T cells, terminal differentiation of effector cells in primary responses, and aspects of memory recall responses. IL-2 also delivers essential signals for thymic development of regulatory T (Treg) cells and later to promote their homeostasis and function. Each of these outcomes on T effector and Treg cells requires distinct amounts of IL-2R signaling, with low IL-2R signaling sufficient for many key aspects of Treg cells. Thus, tolerance is readily maintained and favored with limited IL-2.