ASK1 is essential for sustained activation of JNK and p38 MAP kinases and apoptosis. This study shows that in ASK1-deficient mice, TNF- and H2O2-induced sustained JNK and p38 activation is lost, and ASK1-deficient cells are resistant to TNF- and H2O2-induced apoptosis. TNF, but not Fas-induced apoptosis, requires ROS-dependent activation of the ASK1-JNK/p38 pathway. ASK1 is selectively required for TNF- and oxidative stress-induced sustained JNK/p38 activation and apoptosis. The study demonstrates that ASK1 is required for TNF- and oxidative stress-induced sustained activation of JNK/p38 and apoptosis, but not for Fas-induced apoptosis. The ASK1-JNK/p38 pathway is involved in ROS-dependent apoptosis, while the FADD-caspase 8 pathway is involved in ROS-independent apoptosis. Reintroduction of ASK1 restores TNF-induced JNK/p38 activation and apoptosis in ASK1-deficient cells. These findings suggest that the ASK1-JNK/p38 pathway is a key mediator of TNF- and oxidative stress-induced apoptosis.ASK1 is essential for sustained activation of JNK and p38 MAP kinases and apoptosis. This study shows that in ASK1-deficient mice, TNF- and H2O2-induced sustained JNK and p38 activation is lost, and ASK1-deficient cells are resistant to TNF- and H2O2-induced apoptosis. TNF, but not Fas-induced apoptosis, requires ROS-dependent activation of the ASK1-JNK/p38 pathway. ASK1 is selectively required for TNF- and oxidative stress-induced sustained JNK/p38 activation and apoptosis. The study demonstrates that ASK1 is required for TNF- and oxidative stress-induced sustained activation of JNK/p38 and apoptosis, but not for Fas-induced apoptosis. The ASK1-JNK/p38 pathway is involved in ROS-dependent apoptosis, while the FADD-caspase 8 pathway is involved in ROS-independent apoptosis. Reintroduction of ASK1 restores TNF-induced JNK/p38 activation and apoptosis in ASK1-deficient cells. These findings suggest that the ASK1-JNK/p38 pathway is a key mediator of TNF- and oxidative stress-induced apoptosis.