Photocyclization of carbonyl compounds via remote hydrogen transfer to carbonyl oxygen through medium-sized transition states are summarized on the basis of the mode of the transfer. Remote hydrogen migrates either as a hydrogen atom in direct abstraction by an excited carbonyl group or as a proton from the charge-transfer state produced through interaction between the excited carbonyl and a hetero atom, such as nitrogen. Stereochemistry of 1,8-biradical cyclization and the model for prediction of possible mode of hydrogen abstraction are also described.