The kinetics, mechanism, and thermodynamics of homogeneous cationic polymerization of cyclic acetals are discussed mainly on the basis of studies of 1,3-dioxolane polymerization. Synthetic applications of cationic polymerization of cyclic acetals are indicated. Heterogeneous cationic homo- and copolymerization of 1,3,5-trioxane are discussed in more detail because these processes are used on an industrial scale to manufacture polyacetal (polyformaldehyde) thermoplastics.