The ability to turn on and off specific cell types and neural pathways in the brain, in a temporally precise fashion, has begun to enable the ability to test the sufficiency and necessity of particular neural activity patterns, and particular neural circuits, in the generation of normal and abnormal neural computations and behaviors by the brain. Over the last 5 years, a number of naturally occurring light-activated ion pumps and light-activated ion channels have been shown, upon genetic expression in specific neuron classes, to enable the voltage (and internal ionic composition) of those neurons to be controlled by light in a temporally precise fashion, without the need for chemical cofactors. In this chapter, we review three major classes of such genetically encoded “optogenetic” microbial opsins—light-gated ion channels such as channelrhodopsins, light-driven chloride pumps such as halorhodopsins, and light-driven proton pumps such as archaerhodopsins—that are in widespread use for mediating optical activation and silencing of neurons in species from Caenorhabditis elegans to nonhuman primates. We discuss the properties of these molecules— including their membrane expression, conductances, photocycle properties, ion selectivity, and action spectra—as well as genetic strategies for delivering these genes to neurons in different species, and hardware for performing light delivery in a diversity of settings. In the future, these molecules not only will continue to enable cutting-edge science but may also support a new generation of optical prosthetics for treating brain disorders.