An extension of atom transfer radical addition, ATRA, to atom transfer radical polymerization, ATRP, provided a new and efficient way to conduct controlled/"living" radical polymerization. By using a simple alkyl halide, R-X CX = Cl and Br), as an initiator and a transition metal species complexed by suitable ligand(s), M(t)(n)/L(x), e.g., CuX/2,2’-bipyridine, as a catalyst, ATRP of vinyl monomers such as styrenes and (meth)acrylates proceeded in a living fashion, yielding polymers with degrees of polymerization predetermined by Delta[M]/[I](0) up to M(n) approximate to 10(5) and low polydispersities, 1.1 < M(w)/M(n) < 1.5. The participation of free radical intermediates was supported by analysis of the end groups and the stereochemistry of the polymerization. The general principle and the mechanism of ATRP are elucidated. Various factors affecting the ATRP process are discussed.