The Original Six, expansion and the WHA.

For 25 seasons the NHL had six teams. In 1967 it doubled in a single summer, a rival league arrived in 1972, and by 2021 the NHL had grown to 32 clubs. Much of this history rests on Wikipedia, and the page says so where it does.

The Original Six, 1942 to 1967

From the 1942-43 season until 1967 the NHL had the same six teams for 25 seasons: the Boston Bruins, Chicago Black Hawks, Detroit Red Wings, Montreal Canadiens, New York Rangers and Toronto Maple Leafs. That is Wikipedia's account, and Wikipedia also points out that the name is not literal. Only Montreal and Toronto were members in the league's first season of 1917-18, and the phrase "original six" is recorded only from about 1966, according to a search summary of the same article.

The era belonged to two clubs. Wikipedia counts the Cups won from 1942-43 to 1967 like this:

Team	Stanley Cups, 1942-43 to 1967
Montreal Canadiens	10
Toronto Maple Leafs	9
Detroit Red Wings	5
Chicago Black Hawks	1 (1961)
New York Rangers	0

Montreal's five Cups in a row from 1956 to 1960 remain the record, according to the Hockey Hall of Fame. The last Cup of the era went to Toronto, who beat Montreal in six games in the 1967 final; Wikipedia notes Toronto has not reached a final since.

Players had little freedom of movement. According to Wikipedia, teams held exclusive rights to prospects within 50 miles (80 km) of their home rinks and sponsored junior clubs, tying players to one organisation from a young age. The NHL Players' Association was formed in 1967.

The 1967 expansion

For the 1967-68 season the NHL added six clubs at once, doubling to 12 teams: the Los Angeles Kings, Minnesota North Stars, Oakland Seals (later the California Golden Seals), Philadelphia Flyers, Pittsburgh Penguins and St. Louis Blues. The new clubs formed a West Division and the existing six an East Division. Wikipedia gives two motives: the hope of a United States network television deal, and fear that the Western Hockey League would seek a claim on the Stanley Cup.

All six new teams were American, and Wikipedia says Canadian objections were part of the reason Vancouver and Buffalo were added in 1970. The New York Islanders and the Atlanta Flames joined in 1972, partly to keep a new rival league out of those cities, and Washington and Kansas City followed in 1974.

The World Hockey Association, 1972 to 1979

The rival was the World Hockey Association (WHA). Everything in this section comes from Wikipedia's WHA articles unless another source is named.

Dennis Murphy and Gary Davidson founded the WHA in 1971. It began play on 11 October 1972, when the Alberta Oilers beat the Ottawa Nationals.
Its headline signing was Bobby Hull, who moved from the Chicago Black Hawks to the Winnipeg Jets in 1972.
In November 1972 Judge A. Leon Higginbotham Jr. issued an injunction against the NHL's reserve clause, ending the league's hold on major-league players.
Gordie Howe came out of retirement to play alongside his sons Mark and Marty with the Houston Aeros. Wikipedia's article gives both 1973 and 1974 for this; NHL.com says Howe played in the WHA with his sons from 1973.
From 1974-75 the WHA signed European players in numbers, among them Anders Hedberg and Ulf Nilsson. Vaclav Nedomansky, in 1974, was the first Czechoslovak to defect to North America to play hockey.
Wayne Gretzky played for the Indianapolis Racers in 1978-79 before moving to Edmonton.
The WHA used sudden-death overtime in the regular season before the NHL, which adopted a five-minute sudden-death overtime in 1983-84.

The 1979 merger, or expansion

An NHL Board of Governors vote on 8 March 1979 fell one vote short of the three-quarters majority needed to admit WHA teams. A second vote on 22 March 1979 passed after Montreal and Vancouver changed their votes. The WHA played its last game on 20 May 1979, when Winnipeg beat Edmonton for a third Avco World Trophy, and on 22 June 1979 the NHL admitted Edmonton, Hartford (formerly New England), Quebec and Winnipeg for 1979-80. According to a search summary of Wikipedia, the NHL officially called this an expansion rather than a merger, a description critics dispute.

Of the four, only Edmonton is still in its original city under its original name. Quebec moved to Denver, Winnipeg to Phoenix and Hartford to Raleigh as the Hurricanes. Mark Messier was the last WHA veteran to play in the NHL; his final NHL game was on 3 April 2004.

To 32 teams

Wikipedia's NHL article records the league growing to 30 teams between 1991 and 2000, the last two of that wave being Minnesota and Columbus in 2000. The Vegas Golden Knights joined for 2017-18 and the Seattle Kraken for 2021-22. In April 2024 the Arizona Coyotes franchise was deactivated and its hockey assets, including the roster and draft picks, went to a new Utah franchise. NHL.com says Utah played its first regular-season game on 8 October 2024 as the Utah Hockey Club and had taken the name Utah Mammoth by May 2025. The league now has 32 teams, 25 in the United States and 7 in Canada, according to Wikipedia.

Work stoppages

Wikipedia lists four interruptions in the modern era:

A 10-day players' strike in April 1992.
A 1994-95 lockout that cut the season to 48 games.
The 2004-05 lockout, which lasted 310 days and cancelled the whole season, so no Stanley Cup was awarded in 2005.
A 2012-13 lockout that began on 16 September 2012; a 48-game season started on 19 January 2013.

The 2026-27 season

NHL.com announced in July 2026 that each team plays 84 regular-season games in 2026-27, up from 82, for 1,344 games in all. Bleacher Report calls it the first increase in season length since 1993-94. The season began on 29 September 2026 and is in progress as of October 2026. Its structure and the current teams are on the NHL page.

Sources