Hydro One Limited is an electricity transmission and distribution utility serving the Canadian province of Ontario. Hydro One traces its history to the early 20th century and the establishment of the Hydro-Electric Power Commission of Ontario. In October 1998, the provincial legislature passed the Energy Competition Act which restructured Ontario Hydro into separate entities responsible for electrical generation, transmission/delivery, and price management with a final goal of total privatization.

Ontario Hydro, established in 1906 as the Hydro-Electric Power Commission of Ontario, was a publicly owned electricity utility in the Province of Ontario. It was formed to build transmission lines to supply municipal utilities with electricity generated by private companies already operating at Niagara Falls, and soon developed its own generation resources by buying private generation stations and becoming a major designer and builder of new stations. As most of the readily developed hydroelectric sites became exploited, the corporation expanded into building coal-fired generation and then nuclear-powered facilities. Renamed as "Ontario Hydro" in 1974, by the 1990s it had become one of the largest, fully integrated electricity corporations in North America.