Book excerpt: The spread of weapons of mass destruction poses one of the greatest threats to international peace and security in modern times--the specter of nuclear, chemical, and biological weapons looms over relations among many countries. The September 11 tragedy and other terrorist attacks have been painful warnings about gaps in nonproliferation policies and regimes, specifically with regard to nonstate actors. In this volume, experts in nonproliferation studies examine challenges faced by the international community and propose directions for national and international policy making and lawmaking. The first group of essays outlines the primary threats posed by WMD proliferation and terrorism. Essays in the second section analyze existing treaties and other normative regimes, including the Nuclear Non-Proliferation Treaty and the Chemical Weapons and Biological Weapons Conventions, and recommend ways to address the challenges to their effectiveness. Essays in part three examine the shift some states have made away from nonproliferation treaties and regimes toward more forceful and proactive policies of counterproliferation, such as the Proliferation Security Initiative, which coordinates efforts to search and seize suspect shipments of WMD-related materials.