The Defense Department's 2010 Quadrennial Defense Review expressed concerns about emerging chemical and biological (CB) weapon agents and the ability of U.S. defenses to counter them. Scientific advances that facilitate the development of new and novel CB agents and the difficulties uncovering such work suggest that adversary programs could acquire new CB agents years before U.S. defense planners recognize those agents. Once these CB agents are recognized as threats, the United States will probably need many more years to establish a comprehensive defense against them, and even these defenses are unlikely to protect the civilians, contractors, and allied military personnel essential to modern U.S. military operations. Such gaps in CB agent defense capabilities pose a potentially serious risk to U.S. military operations. To best mitigate this risk, the U.S. Chemical and Biological Defense Program (CBDP) needs to augment current work with enhanced efforts to dissuade adversary CB agent development and to deter adversary use of new CB weapons. Successful initiatives in dissuasion and deterrence will depend on CB defensive programs that appear dynamic, progressive, and integrated with other Defense Department and national-level efforts in counterproliferation. The CBDP could add a second track to the current agent-specific science and technology effort to focus on the mechanisms of CB agent effects and interactions with the environment. The goal of the resulting robust combination of CBDP defense, dissuasion, and deterrence is to induce great doubts in adversaries about the value of employing any CB agents or developing new CB agents.