In the ten years since the Proliferation Security Initiative (PSI) was proposed, the United States has taken steps to expand the network of states involved, improve interdiction laws, and share procedures with involved states. A majority of this effort has been focused on maritime interdiction since most of the world's trade travels by ship. As the PSI participant states continue to improve their maritime capabilities for detecting, tracking, and interdicting chemical, biological, radiological, and nuclear (CBRN) materials and weapons of mass destruction (WMD) devices and adjust their maritime laws, state and non-state actors involved in proliferating these items will seek alternative transport modes. The most attractive alternative is air transport. The air transport option is quicker than maritime or ground options and leaves less time for interdiction. Vann Van Diepen, the Principal Deputy Assistant Secretary of State for International Security and Nonproliferation, stated that interdicting air proliferators is difficult because "you have a lot less time to detect an activity, characterize it, [and] work with another country to take action against it." To address the increase in WMD proliferation through the air the United States must take measures to enhance its air interdiction capabilities by: modifying international aviation laws; enhancing U.S. military doctrine, and; improving exercises with the international community.This compilation includes a reproduction of the 2019 Worldwide Threat Assessment of the U.S. Intelligence Community.The PSI's strength lies in the fact that it is not a treaty, but rather an activity that allows states to participate as they see fit. More states are willing to sign up to an activity that is in their interest and does not have stringent reporting requirements. However, the PSI's strength is also its weakness. Since states are just participating, different states provide different levels of effort towards these principles. Some