Ring-opening metathesis polymerization (ROMP) involving 8-membered cyclic olefins, such as the parent molecules cis-cyclooctene (COE) and cis,cis-1,5-cyclooctadiene (COD), leads to various polyalkenamers that have broad potential in a variety of fields. The use of different catalysts and diverse functionalized monomers allows for control over molar mass and architecture in the resultant polymers. We review the ROMP of unsubstituted and substituted COE and COD to tune the properties of the resultant homopolymers and copolymers. Starting with a survey of homopolymers, we report the strengths and limitations of the ROMP of functionalized 8-membered ring olefins. Then we show how these monomers are used to control crystallinity, hydrophobicity and morphology, in various types of copolymers.