The concept that maleic anhydride cannot be homopolymerized is so widely accepted that it is cited in most textbooks on polymer science as an illustration of the steric hindrance imposed by 1,2-disubstitution of the double bond. However, in recent years it has been demonstrated that maleic anhydride readily undergoes polymerization under gamma and UV radiation as well as in the presence of free radical catalysts and various organic bases. In some cases the polymer contains cyclopentanone groups, resulting from decarboxylation, or unsaturated ketone groups, resulting from ring opening, in addition to the anhydride groups. Further interesting developments include the proposed participation of excited maleic anhydride in the polymerization process and of poly(maleic anhydride) in the grafting of maleic anhydride on various polymeric substrates.