The pioneering investigations of Levi-Montalcini and coworkers established that nerve growth factor (NGF) is an important physiological regulator of neurons of the peripheral nervous system. No doubt in part because of the dramatic and specific effects on the sympathetic and sensory nervous system resulting from immunochemically depriving rodents of NGF (Levi-Montalcini and Booker 1960; Johnson et al. 1983), the notion that the action of NGF was directed exclusively to these peripheral neurons became firmly entrenched. A functional model which proposed that NGF was produced by targets of sensory and sympathetic innervation, regulating the specificity of axon growth to the target and the survival of neurons which found the correct target, became dogmatically accepted. Over the last few years it has become clear that these concepts are only partially correct. Aided in substantial part by improved methods for characterization of sites of expression of NGF and its receptor, it has been shown that the biology of NGF action is much more complicated than was initially imagined.