This book outlines the various threats that cyber warfare poses to operations in the maritime environment (defined broadly) and the abilities of modern navies to defend against those threats. It explains how navies are organized and equipped for cyber operations and the concepts and doctrine adopted by those navies. This includes not just the U.S. Navy, Marine Corps and Coast Guard, but also the navies of allies (NATO, the Quad), opponents (China, Russia) and others. It also explores the relationship between USN.USMC and USCYBERCOM. Specific issues that the USN and USMC face in conducting defensive and offensive cyber operations include: recruiting, training, and retaining cyber personnel; consolidation (ashore) and distribution (at sea) of command of cyber operations; operational relationship to artificial intelligence (AI); relationship to electromagnetic warfare (EMW) overall; combining cyber with kinetic operations; unique cyber aspects of surface, air, littoral, and undersea warfare; weaponized dependence on space; cyberattacks on naval supply chains; and fleet resilience and cyber security.