The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent.aspects of [2 + 2] photocycloaddition chemistry with,an emphasis on synthetically televant, regio-, and stereoselcctive reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 yearS (i.e., from 1995 to- 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in. sections 2 and 4, whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin;