Retinal, the aldehyde of vitamin A, is the chromophore of the visual pigments. Its photochemistry, linkage with apoprotein and spectral behaviour has been reviewed. The photoisomerization of retinal from 11-cis to all-trans triggers the phototranduction events in both vertebrate and invertebrate visual systems. The back photoisomerization from all-trans to 11-cis retinal, which is physiologically significative only in invertebrate, is part of the complex process of rhodopsin regeneration. Retinal-binding proteins with specific photoisomerase activity, such as cephalopod retinochrome and bee photoisomerase are reviewed and their properties compared.