Studies of the brain promise to be revolutionized by new experimental strategies that harness the combined power of optical techniques and genetics. This seminar will describe three examples of such optogenetic approaches. First, the light-activated ion channel, channelrhodopsin-2, can be genetically targeted to defined populations of nerve cells, allowing light to control neural activity in spatially and temporally precise patterns. Such “photostimulation” allows high-resolution mapping of brain circuitry both in vitro and in living animals. Second, the light-activated chloride pump, halorhodopsin, can similarly be used to “photoinhibit” neuronal activity. Third, the fluorescent chloride indicator protein, clomeleon, permits mapping of the spatiotemporal dynamics of brain inhibitory synaptic circuitry. Using light to both control and monitor neural activity creates unprecedented opportunities to explore brain function, screen pharmaceutical agents, and potentially to use light to ameliorate psychiatric and neurological disorders.