This paper reviews the subject of cyclopolymerization with emphasis on the reaction mechanism and the structures of the resulting polymers. In general, the structures have been postulated on the assumption that the reaction products are the result of thermodynamic control, no consideration being given to the possibility of kinetic control. Consequently, the majority of polymers are believed to contain six-membered ring units, often with little supporting evidence. The amount of unsaturation present and the number of uncyclized units is also the subject of ambiguous evidence.