Carotenoids are familiar to all of us through the orange-red colours of popular foods like oranges, tomatoes and carrots, and the yellow colours of many flowers.

They are also added as colorants to many manufactured foods, drinks and animal feeds, either in the form of natural extracts (e.g. annatto) or as pure compounds manufactured by chemical synthesis. The production of carotenoids by biotechnology is of increasing interest. Carotenoids are essential to plants for photosynthesis, acting in light-harvesting and, especially, in protection against destructive photooxidation. Without carotenoids photosynthesis in an oxygenic atmosphere would be impossible.

However, carotenoids are not simply pigments of terrestrial plants. They occur widely in bacteria, fungi and algae, where they can be useful taxonomic markers. The production of carotenoids in seaweed adds up to hundreds of million tons per year.

Some animals use carotenoids for coloration, especially birds (yellow and red feathers), fish (e.g. goldfish and salmon) and a wide variety of invertebrate animals, where complexation with protein may modify their colour to blue, green or purple.

Carotenoids are important factors in human health. The essential role of beta-carotene and others as the main dietary source of vitamin A has been known for many years. More recently, protective effects of carotenoids against serious disorders such as cancer, heart disease and degenerative eye disease have been recognized, and have stimulated intensive research into the role of carotenoids as antioxidants and as regulators of the immune response system.