Summary Phototropic curvature is seen to varying degrees in all green plants from algae to orchids, and especially in seedlings, where it may assume life‐or‐death importance. It also occurs in a number of fungi. Recent work centers on the photorecptor, its action spectrum, the effector mechanism, and the inter‐relationships with normal growth.The action spectrum for one type of phototropism of the oat seedling appears identical with that for the fungus Phycomyces and lies wholly in the blue and near u.v. It may indicate a carotenoid like lutein or possibly a flavin in non‐aqueous medium. Red light does not cause phototropism but can modify the response to the blue region. By analogy with the very similar phenomenon of geotropism, in which the georeceptor is probably a type of plastid, the photoreceptor is probably a specialized organelle which moves in a light gradient; this may be either one type of plastid or perhaps an unusual crystalline body which we have recently discovered in the oat seedling.The effector mechanism in seedlings, which has been re‐investigated, is the lateral asymmetrical distribution of auxin, the amount of the shaded side becoming increased. Photodestruction of auxin appears to play no essential part. This asymmetrical distribution results from modification of the normal translocation mechanism, which is metabolically controlled. In fungi, however, the effector remains unknown.