Highly active metallocenes and other single site catalysts as well as Grubbs and Schrock metathesis catalytic systems have opened up the possibility to polymerize cycloolefins or to copolymerize them with ethene or propene. The polymers obtained show exciting structures and properties. Cycloolefins such as cyclopentene, cyclooctene, norbornene, and their substituted derivatives are incorporated into the polymer chain either by double bonds or by ring-opening metathesis polymerization. Materials with elastomeric properties or tactic polymers with high glass transition temperatures and melting points are obtained with a wide range of microstructures. For example, cycloolefin copolymers and other homo- and copolymers of norbornene are of great academic and industrial interest because of their properties and applications in optoelectronics, lenses, and coatings.