Signal transduction is one of the most exciting research areas in modern biology as it deals with how information flows from the extracellular environment into a living cell to change its metabolism, genotype and phenotype. With the completion of the genomes of human and several other species, it becomes even more important to elucidate the molecular mechanisms that govern cellular functions. The intracellular signaling network, which is composed of many signaling pathways, regulates most, if not all, fundamental cellular activities, from proliferation to programmed cell death (apoptosis). Over the past decade, many signaling pathways have been “mapped’ out. Nowadays, we know in great detail about how a specific signal is transmitted via specific signaling pathways, from the membrane to the nucleus, to change the functions of a cell. In this book, a group of experts present a comprehensive review of one of such signaling pathways, the JNK signaling pathway.