Organolanthanide complexes such as SmMe(C(5)Me(5))(2)(THF) and [SmH(C(5)M(5))(2)](2) initiate the living polymerizations of epsilon-caprolactone and delta-valerolactone to give high molecular weight polyesters (M(n) > 70 000) with extremely narrow molecular weight distributions, M(w)/M(n) < 1.08, while these initiators are completely inert for the polymerization of beta-propiolactone. In contrast to lanthanide alkyl complexes, lanthanide alkoxides such as SmOEt(C(5)Me(5))(2)(OEt(2)), [YOMe(C5H5)(2)](2), and YOMe(C(5)Me(5))(2)(THF) exhibit fairly good catalytic activity for polymerizations of beta-propiolactone, delta-valerolactone, and epsilon-caprolactone. The initiation mechanisms for the polymerization of lactones by LnMe(C(5)Me(5))(2)(THF) (Ln = Sm, Yb) or LnOR(C(5)R’(5))(2) (Ln = Sm, Y) initiators are discussed on the basis of equimolar reactions between organolanthanides and lactones.