Although the issue of knowledge diffusion has been at the heart of nonproliferation research and policies, no study in the political science field has thus far systematically identified the mechanisms that allow the acquisition and efficient use of specialized knowledge related to bioweapons. This analytical gap has led to the commonly held belief that bioweapons knowledge is easily transferable. Studies of past weapons programs, including the former U.S. and Soviet bioweapons programs, show that gathering the relevant information and expertise required to produce a weapon is not sufficient to guarantee success. The success of a bioweapons program is dependent on intangible factors, such as work organization, program management, structural organization, and social environment, which can enhance the advancement of a program or create obstacles to progress. When assessed within smaller state and terrorist bioweapons programs, such as those of South Africa and the terrorist group Aum Shinrikyo, these intangible factors produce the same constraining effects as in larger programs. More important, intangible factors have a significant effect on covert programs, because clandestinity imposes greater restrictions on knowledge diffusion. By taking into account these intangible factors, analysts and policymakers can improve their threat assessments and develop more effective nonproliferation and counterproliferation policies.