This book investigates the role naval intelligence played in the Union blockade of the Confederacy during the American Civil War and examines intelligence support to blockade operations on the Atlantic coast between 1861-1865. Discussion begins with an overview of intelligence in the age of sail and the Navy department's intelligence system at the beginning of the war. Included is a detailed look at intelligence as information, a process and a system including an examination of period sources and communication methods. It then proceeds to examine the role of intelligence on the blockade, discussing its impact on operations and effectiveness in stopping the fast, steam, and sail-driven Confederate blockade runners. Intelligence played a crucial role in the effectiveness of the blockade despite the fact that the Union was never able to completely interdict all maritime traffic from entering or leaving Southern ports. There were significant problems with intelligence on the blockade, especially in the realm of tactical intelligence and dissemination. This study investigates these problems as well as intelligence successes at a time when naval warfare was undergoing a dramatic transformation.