A host of environmental factors regulate the embryonic development of neurons. In adults, the survival of neurons, the regeneration of damaged axons, and plastic changes in axonal arborization are also controlled by a complex array of environmental cues. An important category of regulatory influences involves target-derived hormone-like peptides act­ ing on neuronal cell surface receptors. Nerve growth factor (NGF) was the first such factor to be characterized (LEVI­ MONTALCINI and HAMBURGER 1953) and has served as the model against which all similar factors are compared. A number of factors with properties similar to NGF have been described with activities toward differing populations of neurons. Many of these factors have been characterized only poorly at the biochemical level. However, several factors have been charac­ terized to the extent that molecular clones are available and complete amino acid sequences are known. These include: three structurally related factors, NGF itself (SCOTT et al. 1983), brain-derived neurotrophic factor (BDNF) (LEIBROCK et al. 1989), and neurotrophin-3 (NT-3) (MAISONPIERRE et al. 1990; HOHN et al. 1990); ciliary neurotrophic factor (CNTF) (LIN et al. 1989) and a second set of structural homologs; acidic and basic fibroblast growth factors (aFGF, bFGF) (ABRAHAM et al. 1986; JAYE et al. 1986). Investigators have cloned the receptors for NGF (JOHNSON et al. 1986; RADEKE et al. 1987) and FGF (LEE et al. 1989).