U.S. federal civilian agency established by the Atomic Energy Act, which was signed into law by President Harry S. Truman on Aug. 1, 1946, to control the development and production of nuclear weapons (nuclear weapon) and to direct the research and development of peaceful uses of nuclear energy. On Dec. 31, 1946, the AEC succeeded the Manhattan Engineer District of the U.S. Army Corps of Engineers (which had developed the atomic bomb during World War II) and thus officially took control of the nation's nuclear program.

The AEC was headed by a five-member board of commissioners, one of whom served as chairman. During the late 1940s and early '50s, the AEC devoted most of its resources to developing and producing nuclear weapons, though it also built several small-scale nuclear-power plants for research purposes. In 1954 the Atomic Energy Act was revised to permit private industry to build nuclear reactors (nuclear reactor) (for electric power), and in 1956 the AEC authorized construction of the world's first two large, privately owned atomic-power plants. Under the chairmanship (1961–71) of Glenn T. Seaborg (Seaborg, Glenn T.), the AEC worked with private industry to develop nuclear fission reactors that were economically competitive with thermal generating plants, and the 1970s witnessed an ever-increasing commercial utilization of nuclear power in the United States.

Though it had virtually created the American nuclear-power industry, the AEC also had to regulate that industry to ensure public health and safety and to safeguard national security. Because these dual roles often conflicted with each other, the U.S. government under the Energy Reorganization Act of 1974 disbanded the AEC and divided its functions between two new agencies: the Nuclear Regulatory Commission (q.v.), which regulates the nuclear-power industry; and the Energy Research and Development Administration, which was disbanded in 1977 when the Department of Energy was created.