The major challenge we face in psychiatry is understanding. Psychiatric disease represents the leading cause of disability worldwide, but major pharmaceutical companies are withdrawing from developing new treatments, and many are shutting down psychiatry programs, a situation with major medical, social and economic implications. Reasons cited include the lack of neural circuit-level understanding of symptom states, which impairs identification of new treatments and development of predictive animal models. Part of the solution to this challenge may include technologies such as optogenetics, which has primary value as a research tool well-suited to probing circuit-level causality in complex behaviors. Identifying which patterns of circuit activity are actually causally involved in eliciting normal or pathological behaviors may provide a new kind of target, around which investigators can screen and build therapies (whether pharmacological, surgical, or electromagnetic).