The central operating principle of these elegant molecular machines for ion flux is their unitary, simple nature: They combine the two tasks of light sensation and ion flux into a single protein (with a bound small organic cofactor) encoded by a single gene (Oesterhelt and Stoeckenius 1971). Because of this unitary operating principle, the diverse and efficient microbial opsins have (in retrospect) always had potential for application to neuroscience. However, this potential remained latent for several decades after discovery; many technical challenges needed to be overcome for this possibility to be realized. It has since been shown that expressing microbial opsin genes in neurons can enable millisecond-precision optical control of genetically targeted neuronal populations in mammals