Ameren Corporation is an American power company created December 31, 1997 by the merger of St. Louis, Missouri's Union Electric Company (formerly NYSE: UEP) and the neighboring Central Illinois Public Service Company (CIPSCO Inc. holding, formerly NYSE: CIP) of Springfield, Illinois. It is now a holding company for several power companies and energy companies. The company is based in St. Louis, serving 2.4 million electric, and 900,000 natural gas customers across 64,000 square miles

Ameren is the holding company for the following:

The Ameren Missouri subsidiary owns Bagnell Dam on the Osage River, which forms the Lake of the Ozarks. Ameren Missouri is responsible for managing water levels on the lake according to federal regulations.

Prior to the formation of Ameren, the first major development in the history of its constituent parts occurred in 1929, when the Bagnell Dam was completed on the Osage River and generated almost 175 megawatts of hydroelectricity for Missouri's Union Electric Company. The dam also created the Lake of the Ozarks with 1,400 miles (2,300 km) of shoreline.

In 1931, Union Electric Light and Power sought additional generating sources (interurbans being one need) and the company began buying power from the Keokuk, IA dam, 150 miles (240 km) north of St. Louis. Union Electric later bought the dam, providing 134 megawatts of hydroelectricity carried over a longer distance than had ever been achieved before.

By the 1950s Union Electric owned gas operations in and around Alton, Illinois, and acquired other utilities to become the third largest distributor of natural gas in Missouri.

In 1952, Ameren's second major constituent, the Central Illinois Public Service Company, became a major pooled energy power distributor with its future Ameren mate, Union Electric Company. The arrangement formed the Midwest Power Pool system. The CIPS Meredosia, Illinois Power Station became a key contributor to the pool, which also included the later Ameren subsidiary Illinois Power Company.

In 1963 Union Electric completed construction of one of the largest pumped storage plants at that time, the then-350-megawatt Taum Sauk Plant, in Reynolds County, Missouri.

In 1984 Union Electric added nuclear energy to the mix, when the Callaway Nuclear Generating Station began providing 1,143 megawatts of power from Callaway County, Missouri.

In 1995 shareholders of both CIPSCO Inc. and of its neighboring utility of twice its size, the S&P 500-listed Union Electric Company, approved the merger of the two companies, which were to then be combined as Ameren Corporation. Both of those former utilities had traded publicly on the New York Stock Exchange, under ticker symbol CIP and UEP, respectively. At the time of the merger, Union Electric had assets of nearly US$600 million, but still carried nearly US$1.8 billion in long-term debt, although down from US$2.5 billion which it had accumulated by the 1980s. CIPSCO had assets of about US$210 million, but still carried nearly half of US$1 billion in long-term debt, which it had also accumulated by the 1980s.

The merger was completed on December 31, 1997, when the two public companies became one, as Ameren Corporation, which then began to trade publicly on the New York Stock Exchange with the ticker symbol AEE.

Following the merger, Union Electric began doing business as AmerenUE, now known as Ameren Missouri. Today, with nine power plants Ameren Missouri serves 1.2 million power customers and 110,000 gas customers, primarily in Missouri, where more than half of its customers reside in the St. Louis metropolitan area. It also served Iowa as well through the mid-1990s, and served adjoining parts of Illinois until 2010.

The former CIPSCO Inc. utility, Central Illinois Public Service Company, became Ameren's other operating company, doing business as AmerenCIPS.

In 2000, Ameren formed the holding company, AmerenEnergy Resources. It contained two further subsidiaries, AmerenEnergy Marketing, and AmerenEnergy Generating.

In 2002, Ameren Corporation announced a voluntary retirement program, which was offered to approximately 1,000 of Ameren's 7,400 current employees, expecting to realize significant long-term savings.

In 2003, Ameren acquired Peoria-based CILCORP, Inc. and its leading subsidiary, Central Illinois Light Company, from AES Corporation. CILCORP had traded on the NYSE with ticker symbol CER prior to its acquisition by AES, and by the mid-1990s had become a member of the S&P Small Cap 600 index. CILCO had been another pioneer utility in the region, which had paid a dividend since 1921. By 1996, it had grown to over US$150 million in assets, and carried US$330 million in long-term debt. Following the 2003 Ameren acquisition, that utility then became the subsidiary AmerenCILCO.

At the end of 2003, Ameren's chairman and chief executive, Charles Mueller, retired and was succeeded in both positions by Gary Rainwater, the company's president and chief operating officer the past two years.

In 2004, Ameren acquired (from Dynegy Inc.) its third partner from the 1952 Midwest Power Pool system, Illinois Power Company. That utility had traded publicly on the NYSE under the ticker symbol IPC through the 1980s, and paid dividends since 1947. As of the late 1980s, the company generated electricity and natural gas, almost entirely from coal plants, with less than 1% fueled from oil and gas. By then, with about $360 million in assets, it carried long-term debt of over US$2 billion. In 1991, Illinois Power became the subsidiary of holding company Illinova Corp., which traded on the NYSE with ticker symbol ILN. Illinova had grown to an S&P Midcap 400 stock by 1996, with over US$415 million in assets, and had brought the IP utility's debt down to US$1.8 billion by then. In a merger completed February 1, 2000, Illinova Corp. became a wholly owned subsidiary of Dynegy Inc. (NYSE: DYN), in which Chevron Corporation also took a 28% stake. Dynegy in turn had been created in June 1998, from the merger of Chevron's natural gas and natural gas liquids businesses with Dynegy's predecessor, NGC Corp. (former ticker NGL). NGC had been an integrated natural gas services company around since 1994. Following the 2004 Ameren acquisition of the IP utility, that subsidiary became AmerenIP.

In December 2004, Ameren announced that Patrick T. Stokes, the president and chief executive officer of Anheuser-Busch Cos., Inc., was elected to the Ameren board of directors.

In 2009, AmerenUE signed an agreement to purchase 102 megawatts (MW) of wind power from phase II of Horizon Wind Energy's Pioneer Prairie Wind Farm in Iowa. That's enough to power 26,000 households. The power AmerenUE is purchasing will tie into the Midwest Independent System Operator (MISO) transmission grid, of which the company is a member, fulfilling AmerenUE's commitment to add 100 megawatts of renewable capacity to serve its Missouri customers by 2010.

On October 1, 2010 Ameren's three Illinois operating companies merged to become Ameren Illinois Company. The merger resulted in a single operating company providing power to most of the southern four-fifths of Illinois. However, the Ameren Illinois service territory is split into three rate zones corresponding to the service territories of the three former Illinois operating companies--Zone I (the former AmerenCIPS), Zone II (the former AmerenCILCO) and Zone III (the former AmerenIP). On the same date, AmerenUE changed its name to Ameren Missouri Company. The former Illinois portion of the AmerenUE territory was transferred to Ameren Illinois

In 2015, Ameren became the first major energy company to open an Innovation Center at the Research Park, University of Illinois at Urbana-Champaign.

In 2016, Ameren was named 10th on Business Insider’s 10 best energy companies to work for in America list. In the report, seventy four percent of employees said their job has high meaning.

As of 2017 Ameren shares are mainly held by institutional investors (Vanguard group, BlackRock, State Street Corporation, and others)

Ameren Missouri owns the Taum Sauk pumped storage plant, which failed on December 14, 2005, causing extensive damage to the east fork of the Black River and to Johnson's Shut-Ins State Park. Consequently, FERC fined Ameren $15 million. The State of Missouri has sued Ameren for actual and punitive damages, alleging Ameren recklessly operated the plant and put financial considerations from sale of power to other companies over safety, maintenance and engineering. The plant was operated by remote control with no one onsite during pumping operations. Rebuilding the upper reservoir of the AmerenUE Taum Sauk pumped-storage hydroelectric plant, cost $450 million and added 635 new jobs to the state's economy, while raising state income by $48 million, according to a study conducted by the Center for Economic & Business Research at Southeast Missouri State University. The upper reservoir was constructed with roller compacted concrete (RCC); and online in 2010, this 54.5-acre (221,000 m) reservoir is the largest RCC dam in North America. The 450-megawatt Taum Sauk pumped storage hydroelectric plant began generating electricity again on April 21, 2010.

Ameren Missouri sponsors the Adopt-the-Shoreline program, which enables people to “adopt” portions of the shoreline for litter control. They also control other shoreline activities in the Lake of the Ozarks such as the regulation of boat docks, seawalls and other structures within the Osage Project boundary on the lake and downstream from Bagnell Dam. The company maintains the Lake and Shoreline Protection Hotline that anyone who wishes can use to report neglected docks or other concerns they may have about the lake.

A predecessor of Ameren operated a coal gasification plant at the corner of Ash and Orr streets in Columbia, Missouri which was subsequently demolished. The gasification process contaminated the soil and some groundwater in the area with potentially cancer-causing chemicals. Ameren has since finished removing contaminated soil as of September 2014.