Singlet state oxygen is formed in living systems as a result of impact of  far UV light, dye sensitized activations of oxygen, and enzymic activation of oxygen. The hazardous features of singlet state oxygen are: removal of spin restriction and increased reactivity with biomolecules, sufficient lifetime to cause damage, and high reactivity with biomolecules with conjugated double bonds. Plants in particular require protection against singlet oxygen, and ß-carotene has evolved a system of 11 conjugated double bonds which makes it the most effective known scavenger of singlet state oxygen. Presumably, the energy transitions required for singlet quenching are readily accommodated. Its demonstrated biological role is protection of plant photosynthetic apparatus against light mediated damage. ß-carotene is not known to be an essential nutrient in animals, yet it nevertheless has a number of protective and beneficial functions. ß-Carotene protects against photooxidative dermatitis in porphyrics. Even in normal individuals ß-carotene administration protects against erythemic response to UVB radiation. In chewers of tobacco products ß-carotene diminishes indices of genotoxicity, and causes regression of pre-malignant leukoplakias. In experimental animals, ß-carotene protects against chemical, transplanted and UVB induced tumors. The reported evidence of b-carotene as a possible anti-neoplastic agent could be related to its ability to scavenge free radical species in addition to singlet oxygen.