Organometallic catalysts play an important role in the polymerization of olefins, dienes and styrene. A great part of the chemical industry is using these catalysts to produce increasing amounts of plastic materials. Catalysts such as metallocenes, half-sandwich, nickel and iron complexes, are able to tailor the microstructure of the polymer chain. New properties are obtained. Metallocenes in combination with methylalumoxane or perfluorinated berates as cocatalyst form extremely active catalysts for the polymerization of olefins and are now used in industrial processes. Meanwhile a great number of symmetric and chiral zirconocenes have been synthesized that give totally different structures of isotactic, syndiotactic, stereoblock, or isoblock polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocenes. It has become possible to polymerize cyclic olefins, like cyclopentene or norbornene, with different zirconocenes or nickel and palladium catalysts without any ring opening reaction. The crystalline polycycloalkenes show extremely high melting points which are between 400 and 600 degreesC. Copolymers of cycloalkenes with ethene are amorphous and transparent with a glass transition temperature between 120 and 160 degreesC and they can be used as materials for optical discs and fibers. Half-sandwich titanocenes are able to catalyze the formation of syndiotactic polystyrene.