Individual cellular behaviors are controlled by extrinsic signals within the environmental milieu. These signals maybe ions, nutrients, hormones, or, pertinent to this review, a collection of proteins generally termed “growth factors.” Growth factors, which may have autocrine, paracrine, or endocrine origins, can be presented to cells in a soluble form from the extracellular fluid, or in an insoluble form from adjacent cells or extracellular matrices. Growth factors operate on cells through specific high-affinity cell-surface receptors that when occupied, produce a series of biochemical reactions that collectively comprise alterations of cell behavior. An important field of research aims to understand how, within tissues, individual and combinations of growth factors control cell behaviors. Most of the present information on growth factors derives from the direct examination of factor influences on cells in culture (Varon et al., 1983; Manthorpe et al., 1989). This chapter will focus on nervous system cells (i.e., neurons and glial cells) and will summarize the ways that in vitro cellular behaviors can be affected by growth regulating factors.