In April 1972, the Soviet Union signed an international treaty banning the development, production, use, and retention of biological weapons. That same year, Soviet Premier Leonid Brezhnev signed a decree vastly expanding the country’s efforts to develop and weaponize bacterial and viral pathogens. Before it ostensibly ended a year after the fall of the Soviet Union, the program constituted the longest and largest of its kind, involving as many as 65,000 scientists, engineers, technicians, and support staff, spread across a maze of civilian research centers, ministries, and agencies. Its products included more than a dozen vaccine-resistant strains, including one that would induce “an illness similar to multiple sclerosis, but with quick results.” All of this was clearly in violation of the 1972 treaty and thus regularly denied by Soviet officials until the very end. Leitenberg and Zilinskas lay out the details in this massive volume, which explores every dimension of the program: its technical aspects, what U.S. and British intelligence knew about it, the role of Warsaw Pact allies, the proliferation risk, and how it compared to the Soviet chemical weapons program. Still, they stress how much cannot be known, including all that remains behind the Russian Ministry of Defense’s sealed doors and lips.