Caspase 3 (CPP32) is a key caspase involved in apoptosis, with its role in various apoptotic processes being context- and stimulus-dependent. This study examines the role of CPP32 in apoptosis using mice, embryonic stem (ES) cells, and mouse embryonic fibroblasts (MEFs) deficient for CPP32. CPP32-deficient mice exhibit reduced viability, defective neuronal apoptosis, and neurological defects. Inactivation of CPP32 reduces apoptosis in various settings, including activation-induced cell death (AICD) of peripheral T cells and chemotherapy-induced apoptosis of oncogenically transformed MEFs. The requirement for CPP32 is stimulus-dependent: in ES cells, CPP32 is necessary for apoptosis following UV- but not γ-irradiation. Conversely, the same stimulus can show tissue-specific dependence on CPP32: TNFα treatment induces normal apoptosis in CPP32-deficient thymocytes but defective apoptosis in oncogenically transformed MEFs. In some settings, CPP32 is required for certain apoptotic events but not others: CPP32-deficient cells undergoing cell death are incapable of chromatin condensation and DNA degradation but display other hallmarks of apoptosis. These results indicate that CPP32 is an essential component in apoptotic events that is remarkably system- and stimulus-dependent. Consequently, drugs that inhibit CPP32 may preferentially disrupt specific forms of cell death. The study also shows that CPP32 is not required for cytotoxic killing of target cells and that its deficiency leads to resistance to apoptosis in certain cell types, such as BM neutrophils. CPP32-deficient ES cells are resistant to UV-irradiation and osmotic shock-induced apoptosis but not to γ-irradiation or heat shock. In oncogenically transformed MEFs, CPP32 deficiency reduces chemosensitivity to apoptotic stimuli. These findings highlight the complexity of apoptotic control in mammalian systems and the context-dependent role of CPP32 in apoptosis. The study demonstrates that CPP32 is essential for specific apoptotic events and that its deficiency can lead to defects in apoptosis in certain cell types. The results suggest that the apoptotic machinery is complex and diverse, with different caspases playing specific roles in different contexts. The study also shows that CPP32 is involved in various cellular processes, including the cleavage of PARP and the redistribution of phosphatidylserine. The findings underscore the importance of CPP32 in apoptosis and the need for targeted therapies that can specifically inhibit certain forms of cell death.Caspase 3 (CPP32) is a key caspase involved in apoptosis, with its role in various apoptotic processes being context- and stimulus-dependent. This study examines the role of CPP32 in apoptosis using mice, embryonic stem (ES) cells, and mouse embryonic fibroblasts (MEFs) deficient for CPP32. CPP32-deficient mice exhibit reduced viability, defective neuronal apoptosis, and neurological defects. Inactivation of CPP32 reduces apoptosis in various settings, including activation-induced cell death (AICD) of peripheral T cells and chemotherapy-induced apoptosis of oncogenically transformed MEFs. The requirement for CPP32 is stimulus-dependent: in ES cells, CPP32 is necessary for apoptosis following UV- but not γ-irradiation. Conversely, the same stimulus can show tissue-specific dependence on CPP32: TNFα treatment induces normal apoptosis in CPP32-deficient thymocytes but defective apoptosis in oncogenically transformed MEFs. In some settings, CPP32 is required for certain apoptotic events but not others: CPP32-deficient cells undergoing cell death are incapable of chromatin condensation and DNA degradation but display other hallmarks of apoptosis. These results indicate that CPP32 is an essential component in apoptotic events that is remarkably system- and stimulus-dependent. Consequently, drugs that inhibit CPP32 may preferentially disrupt specific forms of cell death. The study also shows that CPP32 is not required for cytotoxic killing of target cells and that its deficiency leads to resistance to apoptosis in certain cell types, such as BM neutrophils. CPP32-deficient ES cells are resistant to UV-irradiation and osmotic shock-induced apoptosis but not to γ-irradiation or heat shock. In oncogenically transformed MEFs, CPP32 deficiency reduces chemosensitivity to apoptotic stimuli. These findings highlight the complexity of apoptotic control in mammalian systems and the context-dependent role of CPP32 in apoptosis. The study demonstrates that CPP32 is essential for specific apoptotic events and that its deficiency can lead to defects in apoptosis in certain cell types. The results suggest that the apoptotic machinery is complex and diverse, with different caspases playing specific roles in different contexts. The study also shows that CPP32 is involved in various cellular processes, including the cleavage of PARP and the redistribution of phosphatidylserine. The findings underscore the importance of CPP32 in apoptosis and the need for targeted therapies that can specifically inhibit certain forms of cell death.