Within the past decade, organ cultures have reemerged as an important complement to in vivo studies for examining regulatory mechanisms in various neuronal systems. The principle behind organ cultures is to maintain a tissue explant in a state as close as possible to that found in vivo, i.e., to preserve some aspect of the spatial, structural, and/or synaptic organization of the original tissue. As a result, such cultures are often referred to as organotypic and are valuable tools to study organized, yet relatively isolated neuronal systems.