Optogenetic tools enable light-mediated control of cellular excitability and signaling in vivo. By manipulating biological processes, scientists can determine the roles played by these processes in intact biological systems, such as the brain. Such cellular-level control has greatly affected basic science. Here, we discuss how optogenetic tools might be translated into clinical impact through identification of new molecular and circuit-level targets and provide temporally precise interventions for defined biochemical or cellular events.