AbstractThe effect of light on the activity of herbicides within the cell is reviewed. The herbicides are grouped according to the way their phytotoxicity is influenced by light.The largest group, which includes the ureas, triazines and uracils, interfere with photosynthesis by blocking the Hill reaction. Lack of carbohydrates and other manifestations of toxicity appear in the light and these are discussed. The bipyridylium quaternary salts, diquat and paraquat, are not phytotoxic until a free radical is formed utilising light energy via photosynthesis. The free radical is re‐oxidised by molecular oxygen, and a series of reactions are started leading to the formation of peroxide radicals which subsequently disrupts the cell.The phytotoxicity of 2,4‐dinitrpphenol and related compounds is reduced by light. This type of compound combined with a herbicide which inhibits the Hill reaction and thus photosynthetic phosphorylation may act synergistically and mixtures of this type are discus