An overview of the optical methods available to modulate the cellular activity in cell cultures and biological tissues is presented, with a focus on the use of exogenous functional materials that absorb electromagnetic radiation and transduce it into a secondary stimulus for cell excitation, with high temporal and spatial resolution. Both organic and inorganic materials are critically evaluated, for in vitro and in vivo applications. Finally, as a direct practical application of optical-stimulation techniques, the most recent results in the realization of artificial visual implants are discussed.