We have been adapting genetically-encoded reagents from nature and engineering them in order to sensitize cellular processes to being controlled by light. One of our areas of innovation is in creating molecular tools that, when genetically targeted to specific neurons, allow them to be silenced by brief pulses of light. In this way, the necessity of a neural pathway, cell type, or brain region in generating a given behavior, neural computation, or pathology can be assessed, in a time-resolved fashion. Furthermore, these ultraprecise tools may empower a new generation of optical control prosthetics. The reagent halorhodopsin from N. pharaonis (Halo/NpHR), for example, hyperpolarizes cells in which it is expressed, in response to pulses of yellow/orange light. The reagent Arch (archaerhodopsin-3 from H. sodomense) enables currents ~an order of magnitude bigger than those of Halo. The reagent Mac (the opsin from L. maculans) enables silencing in response to blue light, which alongside the other reagents enables multi-color silencing.