Neurotrophic factors are important regulators of the development and maintenance of vertebrate nervous system. During the development of the nervous system, neuronal populations undergo a process of naturally occurring cell death at a time when their axons are innervating target areas. This mechanism ensures a balance between the size of an innervating population and the size of its target territory. Nerve growth factor (NGF) was the first neurotrophic factor to be discovered. More recent work has shown that other molecules are secreted by target organs that regulate neuronal survival and differentiation. This chapter focuses on neurotrophins with brief descriptions of the actions of these other molecules.