The core effectors of apoptosis encompass proteolytic enzymes of the caspase family, which reside as latent precursors in most nucleated metazoan cells. A majority of studies on apoptosis are based on the assumption that caspase precursors are activated by cleavage, a common mechanism for most protease zymogen activations. Although this appears to be true for the executioner caspases, recent research points to a distinct activation mechanism for the initiator caspases that trigger the apoptotic pathways. This mechanism is proximity-induced dimerization without cleavage, and its elucidation has led to the revision of concepts of feedback regulation of apoptosis.