Modulation of the nervous system by electrical or chemical means (neuromodulation) is becoming increasingly sophisticated, with application to a growing number of neurological diseases. However, both chemical and electrical neuromodulation are limited in their specificity. Electrical stimulation, for example, indiscriminately activates different neuronal populations within the electrical field, leading to side effects that can limit efficacy. The delivery of genes that encode proteins capable of conveying light sensitivity to neurons has provided a tool that may overcome some of the limitations of traditional neuromodulation techniques. Activation or inhibition of specific neuronal populations with different wavelengths of light opens up possibilities for modulating neural circuits with previously unimagined levels of precision. We briefly review this new technology, illustrating its advantages and potential applications.