A novel radical 1,2-formylfunctionalization of alkenes involving 1,2(4,5)-formyl migration triggered by addition of various carbon- and heteroatom-centered radicals to alkenes has been developed for the first time, thus providing straightforward access to diverse β-functionalized aldehydes with good efficiency, remarkable selectivity, and excellent functional group tolerance. Analogous transformations mediated by a keto-carbonyl migration have also been effected under similar conditions. This method was used to access ring systems including various benzannulated nine-, ten-, and eleven-membered rings, complex 6-5(6,7)-6(5) fused rings, and bridged rings with diverse functionalities.