Neuronal cell death is thought to be an important phenomenon not only for the developmental but also for the post-matured stages of the PNS and CNS neurons. In the developmental stages, massive neuronal death is observed at specific periods, e.g. the immediately after the arrival of axons to the postsynaptic target fields. This phenomenon is precisely programmed in the developing nervous system, so that they are called physiological or naturally occurring neuronal death. It has been shown that this programmed neuronal death is mediated by apoptotic death cascade, as follows: 1) neurotrophic agents, such as nerve growth factor, or afferent electrical input prevent neuronal death; 2) the macromolecular synthesis inhibitors, such as cycloheximide or actinomycin-D, prevent cell death; 3) DNA fragmentation is detected during the course of cell death; 4) variations of apoptotic morphology are observed in programmed neuronal death. Furthermore, recent studies also suggest that such apoptotic neuronal death occurs in the matured nervous system, e.g. neuro-degenerative diseases, such as Alzheimer's and Parkinson's diseases, or aging. Thus, it is expected that the study of apoptosis in the nervous system give us useful clues to know how the neuronal network are constructed in the development and why the neurodegenerative disease and brain aging occur.