SUMMARYPhotosynthesis often reaches its maximal rate only after an initial induction period of some minutes. Simple induction phenomena in whole plants and intact chloroplasts have common features which imply that they are brought about by the same mechanism. The Osterhout–Haas Hypothesis explains induction in terms of light activation of a catalyst and building up of intermediates. This proposal is examined in the light of contemporary work and it is concluded that the major factor is the accumulation of metabolites brought about by the autocatalytic action of the Benson‐Calvin cycle. Light activation of ribulose diphosphate carboxylase is seen as an important factor in autocatalysis but normally too rapid a process, in itself, to account for delays of several minutes.