This booklet is one of the "Understanding the Atom" Series. Consisting of three sections, it is an account of the development of the first nuclear reactor by a team of scientists led by Enrico Farmi. The first section briefly reviews the early work on nuclear fission and neutron emission, the impact of Einstein's letter to President Roosevelt, the establishment of the Manhattan Engineer District, and the preliminary work on the design and construction of the first atomic pile at Chicago. The section ends with an hour-by-hour description of the experiment culminating in the achievement of the first self-sustaining nuclear reaction on December 2, 1942. The second section consists of Fermi's own story of the first atomic chain reaction, written in observance of the tenth anniversary of the event. The third section is an account by Fermi's wife of a party to celebrate the success of the experiment. Her narrative reveals the great secrecy surrounding the work of Fermi's team. The booklet has numerous photographs of the scientists involved in the project, sketches of the first atomic pile and a photograph of a graph indicating the neutron intensity during various stages of the operation of the atomic pile. Bibliography. (LC)