Caspase-8 plays the role of initiator in the caspase cascade and is a key molecule in death receptor-induced apoptotic pathways. To investigate the physiological roles of caspase-8 in vivo, we have generated caspase-8-deficient mice by gene targeting. The first signs of abnormality in homozygous mutant embryos were observed in extraembryonic tissue, the yolk sac. By embryonic day (E) 10.5, the yolk sac vasculature had begun to form inappropriately, and subsequently the mutant embryos displayed a variety of defects in the developing heart and neural tube. As a result, all mutant embryos died at E11. 5. Importantly, homozygous mutant neural and heart defects were rescued by ex vivo whole-embryo culture during E10. 5–E11. 5, suggesting that these defects are most likely secondary to a lack of physiological caspase-8 activity. Taken together, these results suggest that caspase-8 is indispensable for embryonic development.