This book examines how international intelligence cooperation has come to prominence post-9/11 and introduces the main accountability, legal and human rights challenges that it poses. Since the end of the Cold War, the threats that intelligence services are tasked with confronting have become increasingly transnational in nature -- organised crime, the proliferation of weapons of mass destruction and terrorism. The growth of these threats has impelled intelligence services to cooperate with contemporaries in other states to meet these challenges. While cooperation between certain Western states in some areas of intelligence operations (such as signals intelligence) is longstanding, since 9/11 there has been an exponential increase in both their scope and scale. This edited volume explores not only the challenges to accountability presented by international intelligence cooperation but also possible solutions for strengthening accountability for activities that are likely to remain fundamental to the work of intelligence services. The book will be of much interest to students of intelligence studies, security studies, international law, global governance and IR in general.