The expansion era of the National Hockey League (NHL) began when six new teams were added for the 1967–68 season, ending the Original Six era. The six existing teams were grouped into the newly created East Division, and the expansion teams—the Los Angeles Kings, Minnesota North Stars, Oakland Seals, Philadelphia Flyers, Pittsburgh Penguins and St. Louis Blues—formed the West Division.

The NHL added another six teams by 1974 to bring the league to 18 teams. This continued expansion was partially brought about by the creation of the World Hockey Association (WHA), which operated from 1972 until 1979 and sought to compete with the NHL for markets and players. Bobby Hull was the most famous player to defect to the rival league, signing a $2.75 million contract with the Winnipeg Jets. When the WHA ceased operations in 1979, the NHL absorbed four of the league's teams—the Edmonton Oilers, Hartford Whalers, Quebec Nordiques and Winnipeg Jets. This brought the NHL to 21 teams (the Cleveland Barons had ceased operations in 1978), a figure that remained constant until the San Jose Sharks joined as an expansion franchise in 1991.

The NHL became involved in international play, starting with the Summit Series in 1972, which pitted the top Canadian players of the NHL against the top players of the Soviet Union. Canada won the eight-game series four wins to three with one tie. The success of the series led to the creation of the Canada Cup, held five times between 1976 and 1991. NHL teams also faced Soviet League teams that toured North America between 1975 and 1991 in what was known as the Super Series. The fall of the Iron Curtain in 1989 saw many former Soviet-Bloc players stream into the NHL, joining several players who had defected in the 1980s.

This was the highest scoring period in NHL history. It was led in the 1980s by the Edmonton Oilers and Wayne Gretzky, who scored 200 points or more four times, including a current league-record 215 in 1985–86. Gretzky's 92 goals in 1981–82 also remains a league record. No other player in NHL history has scored 200 points, although Mario Lemieux came close in 1988–89 with 199.

Background Expansion had been a major topic of discussion among NHL owners since 1963, when William M. Jennings of the New York Rangers proposed adding two new teams on the West Coast to counter fears that the Western Hockey League intended to compete as a major league. After several years of discussion, the NHL announced in February 1966 that it would expand by six teams, doubling the league's size. The Los Angeles Kings, Minnesota North Stars, California Seals, Philadelphia Flyers, Pittsburgh Penguins and St. Louis Blues began play in the 1967–68 season. They formed the newly created West Division, and the existing teams were grouped into the East Division. The playoff format was constructed so that an established team would face an expansion team in the Stanley Cup Finals. The Clarence S. Campbell Bowl was created in honour of league president Clarence Campbell, and was awarded to the West Division champion.

The new teams were stocked by the NHL's first expansion draft, as each team selected 20 players from the existing franchises. There was much debate over how many players each existing team could protect: the strongest clubs wished to protect more players, while the weaker clubs hoped that protecting fewer players would help improve the balance of competition. Montreal Canadiens manager Sam Pollock's suggestion to allow each team to protect eleven players to start, then add an additional player to their protected list for each player selected in the draft, was ultimately agreed to as a compromise solution. In addition, an "intra-league draft" was held following the 1968 and 1969 seasons to help accelerate the improvement of the expansion teams. Each team protected two goaltenders and fourteen skaters, leaving their remaining players open to be selected by any other team.

Some teams created instant farm systems by buying existing minor league franchises. The Kings bought the Springfield Indians of the American Hockey League the night before the expansion draft, leading the Flyers to purchase the Quebec Aces. Expansion also changed how the amateur draft was handled. The old system, in which franchises sponsored junior teams and players, was abandoned by 1969 when all junior-aged players were made eligible for the entry draft.

Expansion In their inaugural season, the Flyers finished atop the West Division, recording 73 points in 74 games. The California Seals, a pre-season favourite to win the division, changed their name to the Oakland Seals a month into the season. The team disappointed both on the ice and at the gate, finishing last in the NHL with 14 wins. The Blues defeated the Flyers and North Stars to become the first expansion team to play for the Stanley Cup, where they were defeated in four consecutive games by the Canadiens. The Blues reached the finals again in both 1969 and 1970, but were similarly swept in both years, losing to the Canadiens and Boston Bruins, respectively.

On January 13, 1968, North Stars' rookie Bill Masterton became the first and, to date, only player to die as a result of injuries suffered during an NHL game. Early in a game against the Seals, Masterton was checked hard by two players, Ron Harris and Larry Cahan, causing him to flip over backwards and land on his head. He was rushed to hospital with massive head injuries, and died there two days later. The National Hockey League Writers Association presented the league with the Bill Masterton Memorial Trophy later in the season; the trophy is awarded annually to the player who best exemplifies the qualities of perseverance, sportsmanship and dedication to hockey. Following Masterton's death, players slowly began wearing helmets; starting in the 1979–80 season, the league mandated that all players entering the league wear them.

Bobby Orr

In the 1968–69 season, second-year defenceman Bobby Orr scored 21 goals, an NHL record for a defenceman, en route to winning his first of eight consecutive Norris Trophies as the league's top defenceman. At the same time, Orr's teammate, Phil Esposito, became the first player in league history to score 100 points in a season, finishing with 126 points. He was one of three players to break the century mark that year, including Bobby Hull and 41-year-old Gordie Howe.

A gifted scorer, Orr revolutionized the impact defencemen have on the offensive aspect of the game, as blue-liners began to be judged on how well they created goals in addition to how well they prevented them. Orr scored the Stanley-Cup-winning goal in overtime of the fourth game against the Blues in 1970, which gave the Bruins their first championship in 29 years. The goal was highlighted by his famous flight through the air after being tripped up following his shot. Orr twice won the Art Ross Trophy as the NHL's leading scorer, still the only defenceman in NHL history to do so. His plus/minus of +124 in 1970–71 is a league record. Orr signed a five-year contract that paid him $200,000 per season in 1971, the first $1 million contract in league history.

Chronic knee problems plagued Orr throughout his career. In 1972, he tore ligaments in his left knee after a hard hit, leading to the first of six knee operations. He played 12 seasons in the NHL before injuries forced his retirement in 1978. Orr finished with 270 goals and 915 points in 657 games and won the Hart Memorial Trophy as the league's Most Valuable Player three times. The customary three-year post-retirement waiting period for entry into the Hockey Hall of Fame was waived as he was enshrined in 1979.

Buffalo and Vancouver In Canada, there was widespread outrage over the denial of an expansion team to Vancouver in 1967. Three years later, the NHL added a third Canadian team (and the first in Western Canada) when the Vancouver Canucks, formerly of the Western Hockey League, were admitted as an expansion team for the 1970–71 season, along with the Buffalo Sabres, another team whose owners had bid unsuccessfully for an expansion team in 1967. The Canucks were placed in the East Division, despite being on the west coast, while the Chicago Black Hawks were shifted to the West in an attempt to equalize the divisions' strength.

The league also gave the Sabres and Canucks the first two picks in each round of the 1970 NHL amateur draft, offering them a better opportunity to build their rosters than the 1967 expansion teams had. Buffalo's first round pick was Gilbert Perreault, a future Hall of Famer who would play seventeen seasons in Buffalo.

Before the 1971–72 season, Gordie Howe and Jean Beliveau announced their retirements. Beliveau finished his 18-year career with 10 Stanley Cups, 2 Hart Trophies and 1,219 career points; his points total was the second-highest in NHL history. After playing 25 seasons in the NHL, Howe retired as the league's all-time leader in games played, goals, assists and points. Howe was a six-time league scoring champion, and also won six Hart Trophies. Both players had the customary three-year waiting period waived for entry into the Hockey Hall of Fame in acknowledgement of their achievements.

World Hockey Association In 1972, the NHL faced competition from the newly formed World Hockey Association (WHA). The WHA lured many players away from the NHL, including Derek Sanderson, J. C. Tremblay and Ted Green. The WHA's biggest coup was to lure Bobby Hull from the Black Hawks to play for the Winnipeg Jets. Hull signed a 10-year deal: five years as a player for $250,000 per season, and five more for $100,000 per season in a front-office position. It also included a $1 million signing bonus. The deal totaled $2.7 million, and lent instant credibility to the new league. After Hull signed, several other players quickly followed suit. Bernie Parent, Gerry Cheevers, Johnny McKenzie and Rick Ley jumped to the WHA as the NHL suddenly found itself in a war for talent. By the time the 1972–73 WHA season began, 67 players had switched from the NHL to the WHA. Defections continued following the season, and the WHA scored another major coup when it signed Gordie Howe's sons Mark and Marty. The league then convinced Gordie to come out of retirement to play with them on the Houston Aeros.

The NHL tried to block several of the defections. The Bruins attempted to restrain Cheevers and Sanderson from joining the WHA, though a United States federal court refused to prohibit the signings. The Black Hawks were successful in having a restraining order filed against Hull and the Jets pending the outcome of legal action the Black Hawks were taking against the WHA. The WHA was eager for the court action, intending to challenge the legality of the reserve clause, which bound a player to their NHL team until that team released him. In November 1972, a Philadelphia district court placed an injunction against the NHL, preventing it from enforcing the reserve clause and freeing all players who had restraining orders against them, including Hull, to play with their WHA clubs. The decision effectively ended the NHL's monopoly on major league professional hockey talent.

The NHL also found itself competing with the WHA for markets. Initially, the league had no intention to expand past 14 teams, but the threat the WHA posed changed this. Hoping to keep the WHA out of the newly completed Nassau Veterans Memorial Coliseum in Uniondale, New York and the Omni Coliseum in Atlanta, Georgia, the league hastily announced the creation of the New York Islanders and Atlanta Flames as 1972 expansion teams. Following the 1972–73 season, the NHL announced it was further expanding to 18 teams for the 1974–75 season, adding the Kansas City Scouts and Washington Capitals, which marked the end of the league's first major expansion period. In just eight years, the NHL had tripled in size to 18 teams.

Summit Series