Highly active metallocenes and other single-site catalysts have opened up the possibility to polymerize cycloolefins or to copolymerize them with ethene or propene. The polymers obtained show interesting structures and properties. The cycloolefins such as cyclopentene, norbornene, and their substituted derivatives are incorporated into the polymer chain either by addition or metathesis. An overview of copolymers obtained via addition copolymerization is here given. Materials with elastomeric properties or tactic polymers with high glass transitions and melting points can be obtained, depending on the wide range of different microstructures. Cycloolefin copolymers and homo- and copolymers of norbornene, in particula, are of great academic and industrial interest because of their properties and applications in optoelectronics, lenses, and coatings