Terrorism is increasingly a transnational threat, requiring transnational responses. Consequently, there are many potential incentives for expanding international cooperation in critical areas of science and technology (S&T) as it applies to developing, producing, and deploying systems for homeland defence and counter-terrorism. Europe has made considerable progress in promoting pan-European cooperation in the area of defence-related S&T, and these experiences and efforts could be useful in expanding transnational cooperation in the area of developing new technologies for combating terrorism. Given the United States' longstanding proclivity towards protectionism in its defence industrial base, it will be more difficult to expand transatlantic collaboration in the area of S&T for homeland defense and counter-terrorism. This Chapter concludes with some recommendations for overcoming these impediments, including reforming export controls, reducing controls over security of supply and information, and encouraging new institutional collaborative S&T initiatives on both sides of the Atlantic Ocean.