is a collective term for methodology that utilizes light-sensitive proteins (harvested from nature, or engineered), or light-switchable chemical compounds (that can modify proteins), in cells or organisms, to render cellular processes controllable by light, or to make them visible. This can extend to the control of neurons in animals, thus evoking or inhibiting neuronal activity, and even affecting animal behavior. Optogenetic methods were first introduced in 2005, and initially led to a revolution in neurosciences. In more recent years, it also expanded to the field of cell biology. The success and broad potential of the approach sparked an interest in development of novel optogenetic tools with more specific properties and for ever increasing applications, in basic science, but potentially also for applications in human therapy.