This book describes the synthesis and characterization of niobium-based metal complexes and their usefulness as catalysts for the polymerization of ethylene, norbornene, beta pinene and vinyl monomers. The polymerization of above described monomers was studied using niobium complexes based on N,N dialkylcarbamates, mixed chloro alkoxides, simple oxygen-containing ligands and the starting material for the preparation of such derivatives, i.e. NbCl5.Niobium N,N-dialkylcarbamates exhibited high activity for polymerization of ethylene when activated with TMA-depleted MAO and 1,2-dichloroethane as activator working at 1 bar ethylene pressure. These novel catalytic systems resulted very active in Ring-opening metathesis polymerization (ROMP) of norbornene (NBE)with the highest activity never reported up to now for any transition metal catalysts. The dinuclear niobium compounds and simple oxygen-containing derivatives of niobium pentachloride were tested for ethylene polymerization and produced satisfactory results.In this context, it was found that easily accessible NbCl5 works as an excellent catalyst for the polymerization of ?-pinene styrene and butylvinyl ethers.