In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca 2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca 2+ is highly compartmentalized. In the outer segment, Ca 2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca 2+ regulates cells’ metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca 2+ entry, buffering, sequestration, release from internal stores and Ca 2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca 2+ homeostasis. We also investigate the various roles played by Ca 2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca 2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca 2+ homeostasis and pathologies leading to retinal dysfunction and blindness.