The Nuclear Weapons Complex (NWC) is a nationwide group of government-owned sites that is responsible for the design, development, production, modification, repair, assemble, disassembly, and testing of all nuclear weapons in the U S stockpile. The NWC has evolved since the beginning of the Manhattan Project in 1942 to meet national security objectives. It has met these objectives by: building the weapons that ended World War II; building a stockpile of weapons that served as a deterrent to the Soviet Union during the Cold War; incorporating new technologies into the stockpile; safely downsizing the stockpile and dismantling the excess nuclear weapons after the Cold War was over, and ensuring that the remaining stockpile is safe and reliable. This book provides a high level summary of this story. It begins in 1905 with Einstein's Special Theory of Relativity and his famous equation, E=mc2, and ends in the year 2001 with a discussion of the post-Cold War challenges for the NWC. It explains how and why the NWC grew from three sites at the start of the Manhattan Project to over fifty sites at the height of the Cold War, to eight sites-three laboratories, four production plants and one test site-after the Cold War ended.