In this review, the contribution of spin chemistry (in particular, magnetic resonance-related chemistry) to the photochemical field is briefly introduced. First, the development of a time-resolved EPR method and its significant application to radical-related physical phenomena and chemical reactions are presented. Second, a reaction-control method by means of electron spin operations is introduced, and several reaction yield-detected magnetic resonance (RYDMR) methods are presented as applications of this concept. One of the most important physical conclusions is the introduction of the concept of ''spin phase relaxation'' termed singlet-triplet (S-T) and triplet-triplet (T-T) dephasing, instead of the traditional concepts of longitudinal (T 1 ) and transversal relaxations (T 2 ). The effects of strong microwave power on the RYDMR spectrum and time-domain data are analyzed according to this concept. Furthermore, a new detection method is introduced, termed ''photoconductivity detected magnetic resonance'' (PCDMR), which is applicable exclusively to the system of charge transfer reactions.