This account describes the work of our group in the selective catalytic reduction of nitrogen oxides (NOx) with ammonia over heterogeneous photocatalysts (photo-SCR) in the past 16 years. We have found that the photo-SCR proceeds over heterogeneous photocatalysts using a gas flow reactor, elucidated the reaction mechanism under UV- and visible-light irradiation by spectroscopic and kinetic studies, and developed a highly active photo-SCR system by improving the photocatalyst material itself and the reaction system with several approaches based on the reaction mechanism.