The mechanism of transduction in the photoreceptor remains the least understood link in the chain of molecular and cellular processes that are responsible for vision. This paper reviews the molecular species and processes involved in the absorption of light and subsequent biochemical responses of the photoreceptor cell. The cells' electrical response and the probable ionic basis of the response are described. We develop the outlines of a mechanistic model that attempts to link biochemical and electrophysiological responses of photoreceptors of the vertebrate retina.