After the sensational discovery of group 4 metallocene catalysts, much effort has been devoted to the study on well-defined transition metal complexes with a view towards acquiring new olefin polymerization catalysts. As a result, many high performance new olefin polymerization catalysts based on both late and early transition metal complexes have been developed. Regarding late transition metal complexes, nickel complexes possessing di-imine ligands produced branched polyethylenes in the absence of co-monomers. The nickel complexes behave as the catalysts for the living polymerization of propylene and 1-hexene at low temperatures. The corresponding palladium complexes bearing the same ligands promoted ethylene / methyl acrylate copolymerization to produce polyolefins bearing polar functional groups. In addition, nickel complexes with phenoxy-imine chelate ligands displayed high tolerance to polar functional groups and, thus, incorporated polar olefins to provide functionalized polyethylenes. Recently, iron complexes possessing diimine-pyridine ligands were found to show very high ethylene polymerization activities comparable to those of group 4 metallocene catalysts. Alternatively, concerning early transition metal complexes, titanium complexes bearing di-amide ligands with methylalumoxane (MAO) as a cocatalyst displayed excellent catalytic performance for the polymerization of higher α-olefins to generate high molecular weight poly-1-hexenes. Using borate as the cocatalyst, the titanium complexes promoted the room-temperature living polymerization of 1 -hexene.  In addition, very recently, group 4 transition metal complexes having phenoxy-imine chelate ligands, named FI Catalysts, were revealed to be excellent catalysts for olefin polymerization including living olefin polymerization. FI Catalysts displayed strikingly high ethylene polymerization activities and produced new polymers such as very low and ultra-high molecular weight polymers, monodisperse polyethylenes, ethylene-propylene copolymers, and highly syndiotactic polypropylenes (sPP). Moreover, FI Catalysts created new block copolymers such as sPP-b-poly(ethylene-co-propylene) diblock copolymer and PE-b-poly(ethylene-co-propylene)-b-sPP triblock copolymer.