The brain is made up of an incredible number of different kinds of neuron, which vary in their shapes, molecular compositions, and connectivity patterns, as well as in how they change in different disease states. Understanding how these different kinds of neuron work together in brain circuits to implement perceptions, emotions, decisions, and actions, and how flaws in specific neuron types result in brain disorders, is an ongoing high priority for neuroscience. Over the last several years we have developed a rapidly-expanding suite of genetically-encoded reagents (e.g., ChR2, Halo, Arch, Mac, and others) that, when expressed in specific neuron types in the nervous system, enable their activities to be powerfully and precisely activated and silenced in response to pulses of light. These tools are in widespread use for analyzing the causal role of defined cell types in normal and pathological brain functions. We have begun to develop hardware to enable complex and distributed neural circuits to be precisely controlled, and for the network-wide impact of a neural control event to be measured using distributed electrodes and fMRI. We discuss our pre-clinical work on translation of such tools to support novel ultraprecise neuromodulation therapies for human patients.