Retinal, a precursor of vitamin A, has been used in foods, cosmetics, pharmaceuticals, nutraceuticals, and animal feed additives. Carotenoid 15,15'-oxygenases, including beta-carotene 15,15'-oxygenases from mammalians, chickens, fruit flies, zebrafishes, the uncultured marine bacterium, and the fungus Fusarium fujikuroi, and apo-carotenoid 15,15'-oxygenases from cyanobacteria produce retinal from carotenoids. In this article, the biochemical properties, reaction mechanism, and substrate specificity of carotenoid oxygenases are reviewed, along with a description of the enzymatic biotransformation of carotenoids to retinal. Retinal producing methods using metabolically engineered cells and uncharacterized proteins are suggested.