When the Cold War abruptly ended, DOE halted most nuclear materials production. In 1995, Congress chartered DOE's Environmental Management Science Program (EMSP) to bring the nation's scientific infrastructure to bear on EM's most difficult, long-term cleanup challenges. The EMSP provides grants to investigators in industry, national laboratories, and universities to undertake research that may help address these cleanup challenges. On several occasions the EMSP has asked the National Academies for advice on developing its research agenda. This report resulted from a 15-month study by an Academies committee on long-term research needs for deactivation and decommissioning (D&D) at DOE sites.