Natural killer (NK) cells mediate target cell lysis by two independent pathways, one involving exocytosis of preformed granules (which contain perforin and granzymes A and B), the other requiring ligation of CD95 on target cells with CD95 (APO-1/Fas) ligand on the effector cell (1 –3 ). Both processes lead to target cell apoptosis and lysis (3 ). Perforin and granzymes A (and B) have been implicated as main contributors to membrane and/or nuclear disintegration of target cells by the secretory pathway (3 ), whereas granzyme B and CD95 induce early DNA fragmentation by activation of multiple cysteine proteases of the caspase cascade, including caspase-8 (FLICE) (4 ) and caspase-3 (CPP32) (5 ). The exact mechanisms governing NK-induced death are far more complicated than delineated herein, and the in vivo relevance of their different components require further study. The roles of KIR and class I major histocompatibility complex (MHC) in the effector/target interaction are described in other chapters of this book. Caspases (Cysteine-Aspart-ases ) are key effector molecules involved in programmed cell death (PCD), although some of them can also participate in other physiological processes such as activation of proinflammatory cytokines. Since the discovery of the prototype death protease Ced-3 in Caenorhabditis elegans (6 ) more than 15 mammalian and invertebrate caspases have been described (7 ).