Light-sensitive proteins encoded in DNA can serve as selective optical interfaces for observing and controlling genetically targeted neurons in functioning circuits, in vitro and in vivo. Light-emitting sensors of neuronal activity (reporting calcium increase, neurotransmitter release, or membrane depolarization) have begun to reveal how information is represented by neuronal assemblies, and how these representations are transformed during the computations that inform behaviour. Light-driven actuators control the electrical activity of central neurons in freely moving animals and establish causal connections between the activities of specific neurons and the expression of particular behaviours. The combination of finely resolved optical field sensing and finely resolved optical field actuation is opening new dimensions for the analysis of the connectivity, dynamics, and plasticity of neuronal circuits, and perhaps even for replacing lost – or designing novel – functionalities.