Caspases are best known for their role in apoptosis. More recently, they have gained prominence as critical mediators of innate immune responses. The so-called 'inflammatory caspases' include human caspase-1, -4, -5 and -12 and murine caspase-1, -11 and -12. Of these, caspase-1 is best characterized and serves as the prototype for our understanding of the processing, activation and function of inflammatory caspases. Like their apoptotic counterparts, inflammatory caspases are produced as inactive zymogens and require activation to become proteolytically active. Caspase-1 is activated within the inflammasome, a large cytosolic protein complex that is induced by a growing number of endogenous, microbial, chemical or environmental stimuli. The importance of caspase-1 in initiating innate immune responses is demonstrated by its role in cleaving pro-IL-1 beta and pro-IL-18 to their biologically active forms. New functions have also been implicated, as these proteases and the mechanisms underlying their activation and regulation emerge as important mediators of human health and disease.