Publisher Summary This chapter discusses several classes of nervous system growth factors focusing on neurotrophic factors. In the peripheral nervous system (PNS), neurotrophic factors are survival factors for several classes of neurons. The availability of limited quantities of nerve growth factor (NGF) in a target-derived manner is essential for the survival of sympathetic neurons of the paravertebral ganglia, and for 70 to 80% of sensory neurons (nociceptors) in the dorsal root ganglia. In addition to regulating neuronal survival and function, neurotrophins modulate neuronal differentiation and synapse function during PNS development. Neurotrophins modulate several neuronal systems in the CNS. During development, brain-derived neurotrophic factor (BDNF) is required for the survival of vestibular neurons and neurotrophin-3 (NT-3) is required for the survival of most cochlear neurons. After injury, several neurotrophins protect injured or degenerating adult nervous system neurons and axons. NGF influences the survival and/or growth of several neuronal populations in the injured adult nervous system. It prevents the degeneration of basal forebrain cholinergic neurons after injury and as a result of normal aging in rats and primates. Other class of nervous system growth factors such as fibroblast growth factors, insulin and the insulin-like growth factors, and glial cell line-derived neurotrophic factor are also presented.