Recent information on the use of transition metal chelates as initiators of free radical polymerization of vinyl monomers is supplied. Chelates of various kinds and of various coordination numbers, derived from transition metal ions such as Mn(II), Mn(III), Co(II), Co(III), Fe(II), Fe(III), Cu(I), and Cu(II) with a multitude of both monomeric and polymeric ligands, are taken into consideration. The efficiency of the individual chelates toward radical polymerization of vinyl monomers under variable reaction conditions are discussed. The co-catalytic activity of some chelates in photopolymerization and peroxide initiated vinyl polymerization is accounted for. The kinetic results and spectroscopic evidence leading to the mechanistic details of individual paths of initiation of polymerization are discussed.