The mechanism of transduction of the light stimulus into an electrical response signal is described for visual cells of vertebrates and invertebrates. Absorption of a photon by a rhodopsin molecule in the photosensory membrane of the visual cell leads to a delayed, relatively large single-photon-evoked response, the elementary excitatory response. The rhodopsin molecule starts an enzyme cascade via a G-protein. This leads to a considerably amplified decrease (vertebrates), or increase (invertebrates) of a large number of intracellular messenger molecules. Binding of these messenger molecules causes the opening of the sodium-preferring ion channels in the plasma membrane. Our present knowledge of the molecular mechanisms involved is described.