The complex sensation of vision begins with the relatively simple photoisomerization of the visual pigment chromophore 11-cis-retinal to its all-trans configuration. This event initiates a series of biochemical reactions that are collectively referred to as phototransduction, which ultimately lead to a change in the electrochemical signaling of the photoreceptor cell. To operate in a wide range of light intensities, however, the phototransduction pathway must allow for adjustments to background light. These take place through physiological adaptation processes that rely primarily on Ca 2 ions. While Ca 2 may modulate some activities directly, it is more often the case that Ca 2 -binding proteins mediate between transient changes in the concentration of Ca 2 and the adaptation processes that are associated with phototransduction. Recently, combined genetic, physiological, and biochemical analyses have yielded new insights about the properties and functions of many phototransduction-specific components, including some novel Ca 2 -binding proteins. Understanding these Ca 2 -binding proteins will provide a more complete picture of visual transduction, including the mechanisms associated with adaptation, and of related degenerative diseases. BioEssays 22:337Β±350, 2000.