Nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 are members of a family of proteins (the neurotrophins) that promote the differentiation, growth, and survival of peripheral and central nervous system neurons. Recently, the trk family of proto-oncogenes has been found to encode signal-transducing receptors for these growth factors. This discovery has important implications for our understanding of the normal function of these factors in the developing and adult nervous system. In this review, we highlight recent advances in neurotrophin research and discuss their relevance to neurologic disease.