The morpho-functional organization correlated to photosensitivity in Cnidaria is that of ocelli and extraocular photoreception. Several examples of the second type of organization are reported. The photosensitivity of the cnidarian Hydra is of the extraocular (neural or dermal) type. The effects of photic stimulation (applied according to various experimental protocols: steady condition; step stimulus; single, twin, or repetitive pulses; different polarities and chromaticities of steady, step and pulse stimulation and different phases of pulse application) on the modulation of various bioelectric events linked to the periodic behaviour of the animal are reviewed. The mechanisms correlated with the photobehaviour of Hydra, as well as the problems still open on the molecular mechanisms of phototransduction, are discussed.