The Hanford Site in southeastern Washington state is widely considered to be the most contaminated place in the Western Hemisphere. From 1944 to 1989, Hanford’s primary mission was to produce plutonium for the United States’ nuclear weapons program. In May 1989, the signing of the Tri-Party Agreement between the US Energy Department, the US Environmental Protection Agency, and the Washington State Department of Ecology marked a shift in Hanford’s mission, from plutonium production to the environmental cleanup of massive amounts of radioactive and chemical wastes. The Tri-Party Agreement laid out a 30-year cleanup plan, but things have not gone according to that plan, and the Hanford cleanup is now expected to take at least 70 years to complete. The author describes progress made during the first quarter-century of the cleanup, work yet to be done, and risks posed by the remaining wastes at Hanford. He explains why the cleanup is taking so long, costing so much, and may be headed for further litigation between two of the three signatories to the 25-year-old Tri-Party Agreement. The lessons of Hanford are that such a complex cleanup is extremely and unavoidably expensive, and that extending the cleanup over many decades increases the risk of a catastrophic event.