Just over two years ago the newest member of the interleukin family of cytokines, IL-18, was molecularly cloned. IL-18 was originally identified as a result of its ability to induce interferon gamma production, however with the advent of its cloning and the production of recombinant protein a number of other biological actions have since been identified. Recently the receptor for IL-18 was also characterised. Due to the structural and biological properties shared between IL-18 and IL-1 and their respective receptors, questions relating to IL-18 activities are being answered at a rapid pace. This article addresses the biology of IL-18 in both disease and non-disease states.