Nerve growth factor (NGF) was discovered and the concept of neurotrophic factors as retrograde messengers was developed in studies on the peripheral nervous system (PNS). Initial descriptions of biological effects of newly discovered grow factors often are limited to PNS neurons, whereas studies on the brain typically are behind by several years. Besides the historical development of the field, the complexity of the brain probably is responsible for the difference in the level of analysis and understanding of the two groups of neurons. In contrast to PNS ganglia, the brain represents a very complex array of different neuronal populations and nonneuronal cells, making it necessary to separate cells or identify them with specific markers when attempting to characterize biological actions of growth factors. One would like to have available markers and cell isolation methods for every defined population of brain neurons and nonneuronal cells. Compared to this goal, the presently available methods seem rather crude and are limited to a few populations of brain cells. This chapter summarizes available techniques that have proven useful to describe growth-factor actions on brain cells.