The caspases are a unique family of cysteine proteases, which cleave proteins next to an aspartate residue. Among all known mammalian proteases, only the serine protease granzyme B has similar substrate specificity. In addition to a central role of caspases in the initiation and execution phases of apoptosis, these enzymes have some other non-apoptotic functions in living cells. During apoptosis, upon activation, caspases cleave specific substrates and thereby mediate many of the typical biochemical and morphological changes in apoptotic cells, such as cell shrinkage, chromatin condensation, DNA fragmentation and plasma-membrane blebbing. Thus, detection of activated caspases can be used as a biochemical marker for apoptosis induced by diverse stimuli in many types of cells.