AbstractThe main‐ and side‐reactions which can occur during cationic ring‐opening polymerizations of heterocyclic monomers are discussed. Based on these general reactions these polymerizations are classified into three groups. (1) Living (or “almost living”) polymerizations provide the possibility to produce polymers with predictable molecular weight, narrow molecular weight distributions and having functional end groups (by initiation and/or end‐capping). These polymerizations also allow the production of block and graft copolymers by sequential monomer additions. (2) Polymerizations which show temporary, i.e. reversible, termination reactions may be used to produce polymers with predictable molecular weights with molecular weight distributions of ≈2. Functional end groups may be introduced by end capping or chain transfer. These polymerizations generally lead to considerable fractions of cyclic oligomers. (3) Polymerizations which show irreversible termination reactions by the formation of stable, branched or (macro)cyclic onium ions. These polymerizations stop at limited conversions and are not suitable for the synthesis of well‐d