The radical cation-induced [2 + 2]- and [4 + 2]-cycloadditions of electron-rich alkenes afford cyclobutanes and cyclohexenes, which are used in the synthesis of natural products. The key intermediate is the alkene radical cation, which is generated from alkene by a one-electron oxidant. These cycloadditions have attracted renewed attention thanks to the recent development of initiators, such as a chemical oxidant, an electrochemical method and photoredox catalysts. This Highlight Review summarizes these initiators for [2 + 2]- and [4 + 2]-cycloadditions that leverage the generation of the alkene radical cation.