An increasing number of phytochemicals, including phenols, terpenoids, polyketides, and alkaloids, are being recognized as photochemically active substances or photosensitizers. These compounds, unlike the photosynthetic pigments or the phytochromes, do not have any known functions in the plant species in which they occur. However, when introduced into other biological systems, e.g., cells or complex organisms, many of them arc extremely toxic in light. The cellular targets and the photochemical processes for some of them have been defined. In one type of process there is cycloaddition of the photosensitizer with a nucleic acid base, e.g., the formation of a photoadduct of a furanocoumarin such as 8-methoxypsoralen with thymine in a nucleic acid. Certain alkaloids including furanoquinolines. β-carbolines, canthinones, and certain furanochromones and chromenes appear to be of this type; they are photogenotoxic, giving rise to gross chromosomal aberrations in light. A second type of photoreaction, inherently bimolecular, often leads to oxidations. A very large group of phytochemicals, particularly the polyacetylenes and their thiophene derivatives, belong here. These compounds, characteristic of the Compositae and about 20 other families of flowering plants as well as basidiomycetous fungi, are powerful photosensitizers whose main targets in the phototoxicity process arc cell membranes. A number of natural derivatives of the chlorophylls are also in this class. The uses of some of these photosensitizers in photochemotherapy are presented. In contrast to these phototoxic reactions there are other photochemical processes occurring in plants whose significance is not yet understood. One of these processes leads to structural change in molecules, e.g., synthesis of vitamin D in plants. A fourth process is the light-directed E→Z (trans–cis) isomerism of cinnamic acid derivatives. This important stereochemical change may account for phototropism and spatial orientation in some plants.