Optogenetics is a technique that uses light to control cells in living tissue. The technique involves genetically engineering cells to express light-sensitive proteins called opsins. Opsins are usually found in bacteria and other microorganisms, but they can also be found in some plants and animals. When opsins are exposed to light, they change shape and activate or inhibit the cells they are in. This allows researchers to control the activity of specific cells with great precision.