This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book examines how and why terrorist groups innovate generally and al-Qaeda-related terrorist plots in Europe more specifically. It explores the issue of innovation and learning more generically both in theory, within specific themes and context of al-Qaeda's influence on this process. The book reviews the evolution of specific al-Qaeda-related plots in three northern European Union (EU) states the United Kingdom, Denmark and Germany where there has been a significant volume of planned failed and executed terrorist plots. It presents the cumulative result of three international workshops on terrorist innovation and learning, held at the Swedish National Defence College in April and June 2012 and in June 2013, where we gathered international expertise from academia and government. The book presents a broad interpretation of the term "innovation" in our effort to identify and elaborate on indications of evolution within terrorists' tradecraft and behavior.