This special article deals with the role of processing enzymes in the generation of bioactive cytokines, particularly IL-1 beta and the novel cytokine IL-18, which was formerly called IFN gamma-inducing factor (IGIF). The "classical" pathways of cytokine processing are described, as well as the importance of alternative cleavage enzymes. The topic of this review also concerns the biology of IL-18. The regulation of IL-18 production, the IL-18 receptor complex, and the biological effects of this novel cytokine are described.