A major challenge in understanding the relationship between neural activity and development, and ultimately behavior, is to control simultaneously the activity of either many neurons belonging to specific subsets or specific regions within individual neurons. Optimally, such a technique should be capable of both switching nerve cells on and off within milliseconds in a non-invasive manner, and inducing depolarizations or hyperpolarizations for periods lasting from milliseconds to many seconds. Specific ion conductances in subcellular compartments must also be controlled to bypass signaling cascades in order to regulate precisely cellular events such as synaptic transmission. Light-activated G-protein-coupled receptors and ion channels, which can be genetically manipulated and targeted to neuronal circuits, have the greatest potential to fulfill these requirements.