The brain is made out of an incredible diversity of cells called neurons, which have different shapes, are made of different molecules, and that change in different ways in diseases. In optogenetics, microbial opsins, natural proteins that convert light into electrical signals, are genetically expressed in neurons. Then, light pulses can be used to turn neurons on, revealing how they trigger behaviors, disease states, or therapeutic effects, or to turn neurons off, revealing what functions or dysfunctions they are necessary for.