Interleukin-1 (IL-1) is primarily an inflammatory cytokine. Biologically, IL-1 is more closely related to tumor necrosis factor (TNF) than any other cytokine or interleukin, although the structure and receptors for IL-1 and TNF are clearly distinct. IL-1 is active in the low pM and fM range and IL-1 receptors (IL-1R) are expressed in most cells, although less than 100 receptors per cell is not an uncommon finding. Based on short-term blockade of IL-1 receptors in humans and animals and IL-1 beta knock-out mice, there is no evidence that IL-1 beta plays a role in development, or normal homeostasis such as metabolism, hematopoiesis, renal and hepatic function or regulation of blood pressure. On the other hand, IL-1 alpha is found constitutively produced by various epithelial cells, keratinocytes of the skin and in the brain. In these locations, IL-1 may contribute to cell growth and repair functions. During inflammation, injury, immunological challenge or infection, IL-1 is produced and because of its multiple biological properties, IL-1 must contribute to disease. Most studies on IL-1 are derived from experiments in which humans or animals are injected with IL-1 or IL-1 is added to cells in vitro. The biological properties of IL-1 suggest that its effects often mimic host responses to infection, inflammation, injury or immunologic challenge. Using specific IL-1 blockade, it is clear IL-1 is playing a critical role in some disease processes. This review will focus on IL-1 as a cytokine of primary and strategic importance to the initiation and progression of inflammatory and infectious diseases.