The function of the long-chain, highly unsaturated carotenoids of higher plants in photoprotection is becoming increasingly well understood, while at the same time their function in other processes, such as light collection, needs to be reexamined. Recent progress in this area has been fueled by more accurate determinations of the photophysical properties of these molecules, as well as extensive characterization of the physiology and ecology of a particular group of carotenoids, those of the xanthophyll cycle, that play a key role in the photoprotection of photosynthesis under environmental stress. The deepoxidized xanthophylls zeaxanthin and antheraxanthin, together with a low pH within the photosynthetic membrane, facilitate the harmless dissipation of excess excitation energy directly within the light-collecting chlorophyll antennae. Evidence for this function as well as current contrasting hypotheses concerning its molecular mechanism are reviewed. In addition, the acclimation of the xanthophyll cycle content and composition of leaves to contrasting environments with different demands for photoprotection is summarized.