The caspase family of proteases play pivotal roles in the execution of apoptosis. a morphologically defined form of cell death observed in a variety of physiological and pathophysiological settings. This review focuses on the biochemical and cell biological aspects of caspases, including their structure, mechanism of activation, intracellular localization, inhibitors, substrates, and precise roles in cell death. Such an overview of the basic properties of caspases should provide insights into the apoptotic mechanism: these insights are crucial for understanding the contribution of deregulated apoptosis in hematological disorders. (C) 1999 The Japanese Society of Hematology.