This booklet is part of the "Understanding the Atom Series," though it is a later edition and not included in the original set of 51 booklets. A basic survey of the principles of nuclear energy and most important applications are provided. These major topics are examined: matter has molecules and atoms, the atom has electrons, the nucleus, visualizing the atom, the chemical elements, energy comparisons, nuclear radiations and their penetration of materials, nuclear energy released, nuclear reactors, and service to man. Numerous photographs and diagrams are utilized. (PR)