The existence of interleukin (IL)-2 has been recognized for over 25 years, and it remains one of the most extensively studied cytokines. Here we present a broad overview of IL-2 history, functional activities, biological sources, regulation and applications to disease treatment. IL-2 exerts a wide spectrum of effects on the immune system, and it plays crucial roles in regulating both immune activation and homeostasis. Both IL-2 and its multipartite receptor are prototypical of the Type I receptor superfamily, and both have been exploited in numerous ways in the clinic. Despite the wealth of information generated about IL-2 from in vitro culture systems, in vivo mouse knockout models, and clinical trials in humans, fascinating new aspects of its functions in the immune system continue to emerge.