National Grid plc is a British multinational electricity and gas utility company headquartered in Warwick, United Kingdom. Its principal activities are in the United Kingdom and Northeastern United States.

National Grid has a primary listing on the London Stock Exchange, and is a constituent of the FTSE 100 Index. It had a market capitalisation of approximately £40.4 billion on 23 September 2016, the twentieth largest of any company, with a primary listing on the London Stock Exchange. It has a secondary listing on the New York Stock Exchange.

Before 1990, both the generation and transmission activities in England and Wales were under the responsibility of Central Electricity Generating Board (CEGB). The present electricity market in the United Kingdom was built upon the breakup of the CEGB into four separate companies in the 1990s.

Its generation (or upstream) activities were transferred to three generating companies—PowerGen, National Power, and Nuclear Electric (later British Energy, eventually EDF Energy)—and its transmission (or downstream) activities to the National Grid Company.

In 1990, the transmission activities of the CEGB were transferred to the National Grid Company plc, which was owned by the twelve regional electricity companies (RECs) through a holding company, National Grid Group plc. The company was first listed on the London Stock Exchange in 1995.

With the beginning of the new millennium, National Grid pursued some major mergers and acquisitions. In 2000, National Grid Group acquired New England Electric System and Eastern Utilities Associates. In January 2002, National Grid Group acquired Niagara Mohawk Power Corporation, a New York State utility.

In October 2002, National Grid Group merged with Lattice Group plc, owner of Transco—the gas distribution business (Lattice demerged from BG Group in 2000). National Grid Group changed its name to National Grid Transco plc. In August 2004, National Grid Transco agreed to sell four of its regional gas distribution networks. The total cash consideration was £5.8 billion. NGT kept ownership of four other distribution networks, which comprise almost half of Great Britain's gas distribution network. In July 2005, National Grid Transco was renamed National Grid plc. On 26 July 2005, National Grid Company was renamed National Grid Electricity Transmission plc, and on 10 October 2005, Transco was renamed National Grid Gas plc.

In February 2006, National Grid announced that it had agreed to buy KeySpan Corporation, a gas distributor and electricity producer in the United States, for $7.3bn (£4.1bn) in cash. Around the same time, National Grid also announced the acquisition of the Southern Union Company subsidiary, New England Gas Company in Rhode Island. The acquisitions of the two natural gas delivery companies materially doubled the size of National Grid's American subsidiary, creating the second largest utility in the United States with more than 8 million customers. The acquisition of KeySpan was completed on 24 August 2007 following government and regulatory approval and endorsement by the shareholders of the two companies.

In May 2007, National Grid formed a joint venture with the Dutch transmission operator TenneT for a 260-kilometre (160 mi) 1,000 MW BritNed DC link between the Isle of Grain in Kent and Maasvlakte, near Rotterdam. The installation of the first section of cable link started on 11 September 2009, whilst the entire 260 km (160 mi) cable was completed in October 2010. The interconnection became operational on 1 April 2011, and by January 2012, electricity flow had mostly been from the Netherlands to the United Kingdom. The BritNed interconnection would serve as a vital link for the foreseeable European super grid project.

In spring 2011, National Grid sold off its services in New Hampshire, after their request to increase gas and electric rates was denied.

In November 2015, it was announced that Steve Holliday, the CEO for ten years, would leave in March 2016 and that John Pettigrew, its executive director who joined National Grid 25 years ago, would succeed him. In June 2016, the Energy Select Committee argued that the company faced too many conflicts of interest, particularly with regard to its ownership of international interconnectors. The committee proposed that the company should be split up.

In December 2016, National Grid agreed to sell a 61% stake in its gas distribution business, to a consortium of Macquarie Infrastructure and Real Assets, Allianz Capital Partners, Hermes Investment Management, CIC Capital Corporation, Qatar Investment Authority, Dalmore Capital and Amber Infrastructure Limited, with a further 14% stake under negotiation. The sale is due to be completed by 31 March 2017, subject to clearance by the European Commission.