Interleukin 15 (IL-15) is a cytokine that reveals multiple actions on immune system. Initially it was thought to resemble IL-2, but there is still growing evidence that IL-15 has multiple, unique properties, in some cases acting opposingly to IL-2. IL-15 influences mechanisms of both innate and acquired immunity. It induces differentiation and proliferation of NK cells, increases their cytotoxic properties and ability of IFN-γ secretion and also serves as chemoattractant for them. IL-15 acts also on macrophages/monocytes as well as on neutrophils. In contrary to IL-2, which induces activation-induced cell death (AICD) of T lymphocytes, IL-15 potently inhibits this process and induces their proliferation. It serves as growth factor for CD8+ memory T lymphocytes. In certain conditions IL-15 causes their antigen-independent activation and enhances cytotoxic properties. IL-15 seems to be essential in early activation of dendritic cells. Surprisingly, in many of its actions, the physiologically active form of IL-15 is thought to be bound to its high-affinity receptor IL-15Rα and presented in trans to effector cells. The current article summarizes actual opinions and findings on basic properties of IL-15, its receptor complex and also its impact on immune system.