The case studies pertain to Islamist militancy in Spain and the United Kingdom, with implications for Western Europe generally. These case studies--which are based on interviews with current or former Islamic terrorists, government officials, journalists, academics, and think-tank researchers--show that terrorists’ learning about the design and execution of attacks is based mostly on hands-on training from other terrorists about how to build bombs, fire weapons, survey potential targets, and perform other terrorism-related activities. This finding minimizes the role of the Internet as a source of knowledge about terrorist strategy and tactics. There is a small capacity for the centralized building and transference of innovative and adaptive tactics beyond the fundamentals of bomb-making, weapons-firing, and hostage-taking that have been the hallmarks of Islamic terrorism. In addition, many terrorist conspiracies are compartmentalized, which makes learning difficult by impeding the free flow of information and direct interaction between novices and more experienced terrorists. Other non-compartmentalized conspiracies limit learning experiences because the same people that survey targets and build bombs also execute the attacks. There is very little division of labor and expertise that could upgrade the quality of strategy and tactics. Even in relatively successful terrorist attacks, such as the 2004 Madrid bombings, sloppy execution enabled investigators to identify and track the conspirators rather quickly, preventing additional attacks and limiting militants’ ability to learn from, act on, and transfer their experience to other Islamic terrorists. 2 figures and 316 notes