Retinoid includes vitamins A and synthetic relatives except provitamins A such as carotenes. In this review, recent developments on the chemistry and physiological functions of retinoid (retinol, retinal and retinoic acid etc.) were surveyed, special emphasis being laid on the following subjects: (1) vision (retinal and photo-sensitivities; molecular mechanism of scotopic and photopic vision). (2) systemic actions of vitamins A (multiple physiological functions and their active intermediates). (3) reproduction and morphogenesis (extracellular-transport proteins, intracellular-cytoplasmic transfer proteins, and intracellular-nuclear transacting factors; RAR and RXR; morphogen). (4) chemical prevention of cancer (synthetic retinoid derived from retinoic acid). (5) biological significance of cis-retinoid.