The study investigates the differential requirement of Caspase 9 (Casp9) in various apoptotic pathways in vivo. Casp9 mutation results in embryonic lethality and defective brain development due to decreased apoptosis. *Casp9*−/− embryonic stem cells (ES) and fibroblasts (MEF) are resistant to multiple apoptotic stimuli, including UV and γ irradiation. *Casp9*−/− thymocytes are resistant to dexamethasone and γ irradiation but sensitive to UV irradiation and α-CD95. These findings indicate the existence of at least four distinct apoptotic pathways in mammalian cells, each differentially utilized by various cell types and stimuli. Casp9 is required for caspase activation and maintenance of mitochondrial membrane potential, but acts downstream of cytochrome c release. The study also identifies novel apoptotic pathways, one solely dependent on Casp9, another on Casp3, and one independent of both. These findings highlight the complexity and specificity of apoptotic pathways in mammalian cells, which have implications for drug design and the treatment of diseases related to apoptosis defects.The study investigates the differential requirement of Caspase 9 (Casp9) in various apoptotic pathways in vivo. Casp9 mutation results in embryonic lethality and defective brain development due to decreased apoptosis. *Casp9*−/− embryonic stem cells (ES) and fibroblasts (MEF) are resistant to multiple apoptotic stimuli, including UV and γ irradiation. *Casp9*−/− thymocytes are resistant to dexamethasone and γ irradiation but sensitive to UV irradiation and α-CD95. These findings indicate the existence of at least four distinct apoptotic pathways in mammalian cells, each differentially utilized by various cell types and stimuli. Casp9 is required for caspase activation and maintenance of mitochondrial membrane potential, but acts downstream of cytochrome c release. The study also identifies novel apoptotic pathways, one solely dependent on Casp9, another on Casp3, and one independent of both. These findings highlight the complexity and specificity of apoptotic pathways in mammalian cells, which have implications for drug design and the treatment of diseases related to apoptosis defects.