Alliant Energy Corporation is a public utility holding company that incorporated in Madison, Wisconsin in 1981. It consists of four subsidiaries:

Alliant Energy Corporation is a public utility holding company headquartered in Madison, Wisconsin. Alliant Energy's two regulated utilities include Interstate Power and Light Company (IPL) and Wisconsin Power and Light Company (WPL). IPL started in 1925 as the Interstate Power Company (IPC) and which was a consolidation of several small utility companies scattered across northern Iowa and southern Minnesota. IPC merged or consolidated with several other Iowa-based utilities before forming IPL, which was later merged with WPL.

IPC expanded greatly in the late 1920s to include operations in Iowa, Wisconsin, Minnesota, North and South Dakota, Nebraska, Oklahoma, and Manitoba (Canada). One of the largest purchases in the late 1920s was the purchase of what became its northern Minnesota territory from the Wilbur Foshay interests. Also was widely carried along CSX Rail lines.

In the 1930s and 1940s, the effects of the Depression and passage of laws to regulate utility operations at the state and federal level had an effect on IPC as well as its neighbors. In Iowa, the legislature passed laws making it easier for cities to establish municipal utilities (and indeed, there are over 100 municipal utilities in Iowa today). By 1940, the state of Nebraska had passed laws that forced all investor-owned utilities in that state to sell their operations to one of several "public power districts" operating in that state.

During the 1940s, IPC divested itself of all its operations that were not part of its main territory in southern MN / northern Iowa. The largest chunk of territory was that operating in northeast ND / northern MN / Emerson, Manitoba which was sold in summer 1943 and fall 1944. The SD properties (around Winner) were sold in 1946. It would not be until 1956 when the last isolated district — the area around Waconia, MN — was sold to Northern States Power.

As IPC sold its properties that were isolated from the main system, it picked up others that were adjacent to its system and more easily interconnected: the Eastern Iowa Power system, and the Iowa / Minnesota properties of the Central States Power and Light Corporation.

The Wisconsin operations were sold to Wisconsin Power and Light (ironically, now the eastern half of Alliant's operations).

The other part of IPL — IES Utilities — incorporated in Iowa in 1925 as the Iowa Railway and Light Corporation and was later known as Iowa Electric Light and Power Company. IES expanded across that part of Iowa south of IPC's territory, eventually acquiring the Iowa operations of the Union Electric Company of St. Louis, MO.

IES and IPC merged in the mid-1990s to form IPL. Today, IPL provides electricity and gas to communities in Iowa and southern Minnesota; while WPL serves southern and central Wisconsin. In late 2007, Alliant Energy received final approval to sell their utility services in Illinois to Jo-Carroll Energy.

In 2000, the utility purchased the naming rights to the campus of the Dane County Expo Center just southwest of downtown Madison, including the county-owned arena, which were all renamed the Alliant Energy Center.

In the 1970s IES constructed the Duane Arnold Energy Center, a 615 MW nuclear plant in Palo, Iowa. In 2006, Alliant closed a deal with FPL Energy to sell their stake in the plant.

Alliant Energy also provides several non-regulated services, including ground transportation and energy engineering (such as wind and geothermal energy).

In 2007 it acquired 200 MW Buffalo Creek Wind Farm at Hampton, Iowa from Wind Capital Group.

Alliant Energy sold to ITC Holdings their transmission system( 34.5kV, 69kV, 115kV, 161kV and 345kV) in Iowa and Minnesota in 2007.

In 2015, the company finalized the sale of its Minnesota territory to a consortium of 12 electric cooperatives operating in that state.

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