A major focus of finance is reducing risk on investments, a goal commonly achieved by dispersing the risk among numerous investors. Sometimes, however, risk dispersion can cause investors to underestimate and under-protect against risk. Risk can even be so widely dispersed that rational investors individually lack the incentive to monitor it. This Article examines the market failures resulting from risk dispersion and analyzes when government regulation may be necessary or appropriate to limit these market failures. The Article also examines how such regulation should be designed, including the extent to which it should limit risk dispersion in the first instance.