Alkene transformation via metal-hydride hydrogen atom transfer (MHAT) has become an effective method in synthetic organic chemistry. One of the main reasons for this is the high chemoselectivity of metal hydrides (e.g., iron and cobalt) toward alkenes (Figure ). Markovnikovselective formation of carbon radicals from alkenes is possible even in the presence of various functional groups on the reaction substrate. Various one-electron reactions can occur using carbon radicals. Thus, it provides excellent tolerance to a variety of functional groups that are otherwise sensitive to acids, bases, heat, and reducing and oxidizing agents, and is an essential tool for the rapid and selective construction and modification of structurally diverse compounds. The regioselectivity of MHAT-type reactions for alkenes usually exhibits the same Markovnikov selectivity as that of reactions with Brønsted acids. We began our research on molecular transformations by the MHAT mechanism approximately 10 years ago and discovered the MHAT/RPC (radical-polar crossover) reaction described below. This review discusses the history of the MHAT mechanism and recent findings during 2016 -2024. Figure The MHAT mechanism was first proposed by Anthracene is selectively reduced to dihydroanthracene using a cobalt carbonyl complex in a mixed gas atmosphere of hydrogen and carbon monoxide. This reaction is believed to occur via the formation of the active species, cobalt hydride, from the cobalt carbonyl complex and molecular hydrogen, leading to MHAT to anthracene. Similarly, Halpern et al. reported the MHAT-type reduction of methylstyrene using a manganese hydride complex. More than 10 years after the report by Halpern, in 1989, Mukaiyama and Isayama reported an alkene hydration reaction using Co(acac)2 and PhSiH3 under an oxygen atmosphere. Neither the presence of cobalt hydride nor the occurrence of the MHAT mechanism was mentioned in this paper; Nojima et al. explained the mechanism of a similar reaction by hydrometalation with cobalt hydride in 2002. Although Drago also reported a similar cobalt-catalyzed reaction in 1982, before that of Mukaiyama and Isayama, the reaction conditions did not include silylhydride and the reaction was thought to occur via a reaction mechanism in which cobalt hydroperoxide was the reaction active species. Today, these cobalt-catalyzed reactions are considered to occur via the MHAT mechanism. The contributions of Shenvi 1, 8 , Norton 9 , Boger 10 , Herzon 11 , and others have been significant in organizing the reaction mechanisms. In particular, when cobalt hydride is the active species, either the MHAT or hydrometalation mechanism is considered as being possible. In this regard, Shenvi et al. favored the MHAT mechanism for the following reasons : 1) the presence of carbon radicals is supported by radical clock and radical capture experiments; 2) reactivity is not significantly affected by the steric and electronic factors of the substrate; 3) hydrometalation of alkyl-substituted double bonds is usually anti-Markovnikov selective; 4) the reaction proceeds easily at room temperature even with sterically hindered substrates to form metal-carbon bonds; and 5) the reaction proceeds without an empty coordination site near the metal-hydride ligand in a planar tetracoordinated metal complex. 6) the reaction proceeds even in the absence of a vacant coordination site near the metal-hydride bond in the square planar pyramidal complex.