The preparation and 1,2-cycloaddition of halogenated ketenes with olefins and other unsaturated compounds is described. Many illustrations of the formation of α-halo- and α,α-dihalocyclobutanones and other 4-membered rings are given. The usefulness of halogenated ketenes in synthesis is demonstrated by some examples of the conversion of dihalo- and alkylhaloketene cycloadducts with cyclopentadiene to tropone derivatives. Further utility is shown by some ring contraction reactions of the cyclobutanones and cyclobutanols to cyclopropane derivatives.