We have developed a set of "optogenetic" reagents: fully genetically encoded reagents that, when targeted to specific cells, enable their physiology to be controlled via light. To confront the three-dimensional complexity of the living brain, enabling the analysis of the circuits that causally drive or support specific neural computations and behaviors, our lab and our collaborators have developed hardware for delivery of light into the brain, enabling control of complexly shaped neural circuits, as well as the ability to combinatorially activate and silence neural activity in distributed neural circuits. We anticipate that these tools will enable the systematic analysis of the brain circuits that mechanistically and causally contribute to specific behaviors and pathologies. We distribute these tools as freely as possible, and host visitors regularly to learn how to use them .