The Union Electric Company of Missouri was an electric power utility that was organized in 1902 and grew to be one of the large U.S. companies listed among the S&P 500. In 1997, its holding company merged with a smaller neighboring utility, Central Illinois Public Service Company through its holding company, CIPSCO Inc., to form Ameren Corporation based in St. Louis, Missouri.

Ameren Corporation is an American power company created December 31, 1997, by the merger of St. Louis, Missouri's Union Electric Company and the neighboring Central Illinois Public Service Company of Springfield, Illinois. It is now a holding company for several power companies and energy companies. The company is based in St. Louis, with 2.4 million electric, and 900,000 natural gas customers across 64,000 square miles in central and eastern Missouri and the southern four-fifths of Illinois by area.