The c-Jun N-terminal kinase (JNK) group of mitogen-activated protein kinases (MAPKs) is activated in mammalian cells by environmental stress, pro-inflammatory cytokines, and mitogenic stimuli. Biochemical and genetic studies demonstrate that JNK regulates the activities of many transcription factors, and that the JNK pathway is required for the regulation of inflammatory responses, cell proliferation, and apoptosis. The involvement of JNK in apoptotic cell death is particularly intriguing, and has been actively studied in recent years. An improved understanding of JNK-mediated apoptotic signaling may provide novel strategies in prevention and treatment of cancers.