The Defense Department is concerned about emerging chemical and biological (CB) weapon agents and the ability of U.S. defenses to counter them. Due to scientific advances that facilitate the development of new and novel CB agents and the fact that uncovering such work will be a difficult intelligence challenge, the Emerging Threat Agent Project (ETAP) undertook a study to examine the challenges of emerging CB agents and propose measures to reduce their risks. The authors conclude that the problem is comprised of two related components. Given the inherent secrecy with which states and other actors will conduct CB agent development, adversary programs could acquire new CB agents years before U.S. defense planners recognize those agents. And, after the U.S. intelligence community recognizes those CB agents as threats, the United States will probably need many more years to establish a comprehensive defense against them. Such gaps in CB agent defense capabilities pose a potentially serious risk to U.S. military operations. To best mitigate this risk, the Chemical and Biological Defense Program (CBDP) of the U.S. Department of Defense 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 counter proliferation. Many of these objectives can be met by adding a second track to the current agent-specific science and technology (S & T) effort, a track that focuses on the mechanisms of CB agent effects and interactions with the environment.