Members of the microbial opsin gene family have emerged recently as crucial tools for optogenetics, a new neuroscience technology. “Opto genetics” can be defined as the integration of optics and genetics to control well-defined events (such as action potentials) within specified cells (such as a targeted class of projection neurons) in living tissues (such as the brains of freely behaving mammals). In this chapter, we focus on the diversity of the microbial opsin genes and the structure‐function properties of their corresponding proteins.