Exelon Corporation is an American Fortune 100 energy company headquartered in Chicago, Illinois. It generates revenues of approximately $34.5 billion and employs approximately 34,000 people. Exelon is the largest electric holding company in the United States by revenue, the largest regulated utility in the United States with approximately 10 million customers, and is also the largest operator of nuclear power plants in the United States. It was created in October 2000 by the merger of PECO Energy Company and Unicom, of Philadelphia and Chicago respectively. Unicom owned Commonwealth Edison. Exelon operates regulated utilities in Illinois, Pennsylvania, Maryland, Delaware, New Jersey, and Washington, DC. In October 2009, Exelon had full or majority ownership of 17 nuclear reactors in 10 nuclear power plants. Exelon has operations and business activities in 47 states, the District of Columbia and Canada and is the largest competitive U.S. power generator with approximately 35,000 megawatts of owned capacity. Exelon merged with Constellation Energy Group in March 2012 and acquired Pepco Holdings in March 2016. Exelon consists of eight main operating subsidiaries with Exelon Generation, a deregulated energy generator, Constellation, a deregulated competitive energy supplier, and six regulated utilities, Commonwealth Edison (Illinois), Philadelphia Electric Company (Pennsylvania), Baltimore Gas and Electric (Maryland), Delmarva Power & Light (Delaware and Maryland), Atlantic City Electric (New Jersey), and Potomac Electric Power Company (Washington, DC and Maryland).

On June 30, 2005, the Federal Energy Regulatory Commission approved the merger of Exelon and Public Service Enterprise Group Inc., a New Jersey utility. Under this merger, Exelon would have become the largest utility in the United States. The two companies later broke off the agreement due to pressure put on the NJ Board of Public Utilities by public interest groups, including New Jersey Citizen Action. The merger sat pending in front of the NJBPU for nineteen months before Exelon concluded that they were fighting a losing battle. On April 28, 2011, Exelon announced a merger with Constellation Energy for $7.9 billion with the combined company owning more than 34 gigawatts of power generation (55 percent nuclear, 24 percent natural gas, 8 percent renewable including hydro, 7 percent oil and 6 percent coal). The merger was completed on March 12, 2012.

In 2012, when announcing the cancellation of new nuclear construction for Victoria County Station, Texas, Exelon stated that economic and market conditions, especially low natural gas prices, made the "construction of new merchant nuclear power plants in competitive markets uneconomical now and for the foreseeable future".

Exelon announced the proposed purchase of Pepco Holdings, Inc on April 30, 2014, for $6.8 billion in an all-cash transaction. The merger was rejected by the District of Columbia Public Service Commission in August 2015, though it was approved by other federal and state regulators. The companies appealled the decision. On March 23, 2016, the merger was approved by the Washington DC Public Service Commission, under a revised set of terms. The merger was completed later that day making Exelon the largest regulated utility in the United States by customer count and total revenue.

PECO Energy, one of Exelon's local power companies, has been involved in two controversies with activists. In the 1970s, activists delayed the opening of nuclear power plants. In 2015, Earth Quaker Action Team began a campaign to pressure PECO to expand the solar power it purchases, and to purchase it locally to create jobs.

Exelon's merger with Pepco faced stiff opposition from community groups and the D.C. Mayor Muriel Bowser.

Exelon is a leading nuclear power plant operator in the United States with over 19,000 megawatts through Exelon Nuclear Partners, a division of Exelon Generation.

In December 2016, the Illinois legislature voted to pass legislation that would ensure continued operations of both the Quad Cities and Clinton nuclear power plants. The two plants create approximately 23 percent of emission-free electricity in Illinois. Passage of the bill and keeping the plants open is expected to preserve around $1.2 billion in annual economic activity within the state. In August 2016, Exelon agreed to purchase the James A. Fitzpatrick Nuclear Power Plant from Entergy Corporation. Exelon formally acquired ownership and operation of James A. Fitzpatrick Nuclear Power Plant on March 31, 2017.

Exelon owns and operates a fossil generation portfolio of nearly 10,000 megawatts (MW).

Exelon has 47 wind projects in ten states, totaling nearly 1,500 megawatts (MW). In 2010 Exelon purchased John Deere Renewables for $860 million. This purchase added 735MW to Exelon's wind energy generation capacity.

Exelon PowerLabs, LLC, a wholly owned subsidiary of Exelon Generation, provides quality laboratory services to Exelon and its subsidiaries, as well as other organizations across multiple industries. Exelon PowerLabs operates at 4 locations nationwide. The Plattsburgh, New York and Madison, Pennsylvania facilities specialize in calibration services; the Wilmington, Illinois facility specializes in failure analysis and component testing, and the Coatesville, Pennsylvania (headquarters) location provides all services. Their quality system is ISO 9001:2000 registered for the Coatesville and Plattsburgh facilities. The Coatesville, Madison and Plattsburgh facilities are also ISO/IEC 17025 accredited through the American Association for Laboratory Accreditation (A2LA). Exelon PowerLabs has been operating since 1911.

Constellation, an Exelon Company, provides energy sales in deregulated markets to business, residential, and government customers across the United States. Their customers include two thirds of Fortune 100 companies.

Exelon is the largest regulated utility in the United States with approximately 10 million customers and a service area covering six states and the District of Columbia.

Atlantic City Electric

Atlantic City Electric is a regulated utility located in New Jersey with 545,000 electric customers.

Baltimore Gas and Electric (BGE) is a regulated utility located in Maryland with 1.25 million+ electric customers and 650,000+ natural gas customers.

Commonwealth Edison (ComEd) is regulated utility located in Illinois with 3.8 million electric customers.

Delmarva Power is a regulated utility located in Delaware and the Eastern Shore of Maryland with 510,000 electric customers over 5,000 square miles of service territory. They provide natural gas to 130,000 customers in northern Delaware.

Philadelphia Electric Company (PECO) is regulated utility located in eastern Pennsylvania with 1.6 million electric customers and over 500,000 natural gas customers.

Potomac Electric Power Company (PEPCO) is a regulated utility in Washington, D.C. that serves 815,000 electric customers in the district and parts of Maryland.

Exelon Transmission Company is specifically focused on the development of power transmission line specifically the RITELine project which involves work in Ohio, Indiana, and Illinois.

Exelon's Political Action Committee (PAC) is EXELONPAC. The company is positioned to profit from "expensive carbon" and has been lobbying for cap and trade of carbon dioxide emissions. "[Former] Exelon CEO John Rowe is a vociferous and longtime advocate of climate change legislation. In 2009, Forbes reported that if the Waxman-Markey climate legislation became law, 'the present value of Exelon's earnings stream would increase by $14 a share, or 28%.'" Executives at the company have close ties to the Obama administration as advisors and fundraisers. Exelon also disclosed multiple contributions to political nonprofit organizations, the largest of which was $290,000 given to the American Energy Alliance — a 501(c)(4) nonprofit with ties to the conservative billionaire brothers Charles and David Koch and led by former Koch Industries lobbyist Thomas Pyle.

In 2005, Exelon was required to pay a $602,000 fine for exceeding the permitted sulfur dioxide emission limit from April to October 2004 at its Cromby Generating Station in Chester County, Pennsylvania.

Exelon and Illinois state officials waited for four years until 2006 before disclosing that Exelon's Braidwood Nuclear Generating Station, a nuclear plant 60 miles southwest of Chicago, had spilled millions of gallons of water containing tritium, a radioactive form of hydrogen, multiple times over a decade. Exelon officials eventually apologized and said the risks from the leak were "minimal", with tritium levels in surrounding wells all found to be below regulatory limits.

In 2009, the Nuclear Regulatory Commission announced its plan for a $65,000 fine against Exelon for permitting its contracted security guards that were guarding its Peach Bottom Nuclear Generating Station, a two-reactor nuclear plant located in Delta, Pennsylvania to sleep on the job. The incidents did not come to light until a videotape of the security guards was leaked to news media. As a result, Exelon terminated the security contract of the Wackenhut security firm that had been involved and now operates its own in-house nuclear security force.