Neurotrophic factors promote maintenance, repair and regeneration of selected neurons in vitro and in vivo. They include nerve growth factor (NGF) and other neurotrophins, ciliary neurotrophic factor (CNTF) and other cytokines, and a number of growth factors. Molecular properties of the trophic proteins and their transducing receptors are increasingly well characterized. Interest in the neurotrophic factors has greatly accelerated over the past decade with accumulating evidence that they play important roles in the adult mammalian CNS as well as in its development. Adult in vivo models for degenerative CNS conditions have demonstrated the ability of exogenous neurotrophic factors (NTFs) to protect against neuronal damage induced by traumatic or chemical experimental lesions, particularly with regard to easily identifiable neurons such as cholinergic and dopaminergic ones. Other models have shown beneficial effects of NTF administration with regard to axonal regeneration inside CNS tissue.