An introduction to Pakistan's ISI, which has been both an essential ally and problematic partner of the United States Forged during the tumultuous aftermath of Partition in 1947, the Inter-Services Intelligence Directorate (ISI) has grown to become the preeminent intelligence service in Pakistan. Its capabilities are comprehensive, its remit covers both foreign and domestic intelligence, and it is one of the most feared and respected agencies of the Global South. Pakistan's ISI provides an up-to-date and detailed introduction to the ISI and its historical evolution. The narrative is rooted in a deep and wide-ranging contextualization of the state of Pakistan and its security environment. The story is one of an agency that grew from humble beginnings into an extremely capable and robust force at the heart of power in the state. The ISI utilizes broad human intelligence networks and employs covert action and support for militants, particularly in its rivalry with India. As a crucial intelligence partner for the West during the Cold War and into the contemporary era, the ISI has been both an essential ally and problematic partner. The shadow of this agency continues to loom over Pakistan's democratic institutions. This book will appeal to students, scholars, and general readers interested in intelligence and the politics and history of South Asia.