The development of nuclear weapons by the Manhattan Project during World War II was one of the most dramatic scientific/technological episodes in human history. This book offers a concise survey of the essential physics concepts underlying fission weapons. It describes the energetics and timescales of fast-neutron chain reactions, why only certain isotopes of uranium and plutonium are suitable for use, how critical mass and bomb yield can be estimated, some of the design details of the Little Boy and Fat Man bombs, and much more.