The Macdonald-Cartier Freeway, more commonly known as Highway 401, is a freeway that runs across southern Ontario, Canada, the longest 400-Series Highway in Ontario, as well as the busiest highway in the world.

Highway 401 begins at Highway 3 in Windsor, Ontario (not at the Michigan border, contrary to popular belief), and ends at the Quebec border in Eastern Ontario, 815 kilometres from its start point in Windsor. Construction began in 1939, but was almost immediately shut down due to the outbreak of World War II. The first section of the highway was opened between Toronto and Oshawa in 1947 and designated Highway 2A. It was officially designated 401 in 1952. The section of the highway that ran through Toronto quickly became an urban commuter road, rather than a long-distance bypass route as was originally planned, leading to extensive traffic jams. This problem was solved to some extent by implementating an Express/Local setup similar to the new Dan Ryan Expressway in Chicago. By 1967 the highway had been expanded from four lanes to no less than 12 lanes through Toronto and Mississauga. Today the entire 401 through the Greater Toronto Area ranges from 10 to 20 lanes! It is widely believes that the 401 is the longest highway in the world with a consecutive section of 12 lanes or more (from Highway 403 in Mississauga to Brock Road in Pickering). An extensive plan is currently underway by the Ministry of Transportation to widen the highway to no less than six lanes for its entire length and to expand the 12-lane Express/Local system as far west as Guelph.

Major freeway junctions are located at Highway 402, Highburry Ave in London, Highway 403, Highway 8, Highway 6, Highway 407, Highway 403, Highway 410, Highway 427, Highway 27, Highway 400, Allen Road and the Don Valley Parkway in Toronto, Highway 404 and Highway 416.

In 1965 Premier John Robarts named the highway the Macdonald-Cartier Freeway, in honour of Sir John A. Macdonald and Sir Georges-Etienne Cartier, two of the most important Fathers of Canadian Confederation.

Today over 400,000 vehicles use it every day, and there are 18 service stations located along the route. It is one of the most important highways in Canada, as it connects the majority of the population of southern Ontario with all the other highways in the province, as well as all the roads that cross the border with the United States. The border crossing at Windsor and Detroit is the busiest in Canada, and although the 401 itself does not extend the last few kilometres into Detroit, it is the only route from Toronto to Windsor. A large majority of trade between Canada and the United States is carried out due to the 401.

It also serves as the principal connection to Montreal and points east (at the Quebec border, it becomes Autoroute 20).

Cities along the route of the highway include Windsor Chatham, London, Woodstock, Kitchener, Cambridge, Guelph Mississauga, Toronto, Oshawa, Cobourg, Belleville, Kingston, and Cornwall.

• Ontario Ministry of Transportation
• Live 401 Traffic Cameras through the Greater Toronto Area