The National Hockey League (NHL) is a professional ice hockey league in North America, with headquarters located in New York City. It is comprised of 30 franchises, 24 from the United States and six from Canada. Every year teams compete for the Stanley Cup, the oldest professional sports trophy in North America.

The league was founded in Montréal, Canada in 1917 as a result of disagreements between owners of the National Hockey Association (NHA, 1909-1917). The rules, trophies and surviving teams of the NHA were continued in the NHL. The first season (1917-1918) showcased four teams: the Montréal Canadiens, Toronto, the Ottawa Senators, and the Montréal Wanderers. The Montréal Wanderers withdrew after just six games when their arena burned down. Toronto won the NHL playoff and competed for the Stanley Cup against the Vancouver Millionaires of the Pacific Coast Hockey Association (PCHA), winning the best-of-five series 3 games to 2. 1925 would be the last time a team outside the NHL won the Stanley Cup, when the Victoria Cougars defeated the Montréal Canadiens.

The 1920s saw expansion of the league and the birth of the first American team, the Boston Bruins in 1924. The Montréal Maroons were added in the same year and by the end of the decade teams also existed in New York, Pittsburgh, Chicago, and Detroit.

Both the Great Depression and World War II took a toll on the league, which was reduced to six teams for the 1942-43 season. These teams became known as the Original Six and would remain constant for 25 seasons. Each franchise survives today (as of the 2009-2010 season), they are: the Montréal Canadiens, Boston Bruins, Toronto Maple Leafs, New York Rangers, Detroit Red Wings, and Chicago Blackhawks.

Competition from the Western Hockey League prompted the NHL to expand prior to the 1967-68 season, adding six new American teams, much to the chagrin of Canadian hockey fans. In 1972 the World Hockey Association (WHA) formed, and the two leagues would end up battling for markets and players until the end of the decade. In 1979 the WHA folded and four of their franchises were absorbed into the NHL: the Winnipeg Jets, Hartford Whalers, Québec Nordiques, and the Edmonton Oilers, which would go on to dominate the league during the 1980s under the leadership of Wayne Gretzky.

Gretzky was traded to the Los Angeles Kings in 1988, a move that significantly improve the popularity of hockey in the United States. The early 90s would see many more expansion teams, and the result is the 30 franchises that exist today.

Labor Disputes
Three league-wide work stoppages have occurred in NHL history between 1992 and 2005. The first only lasted for 10 days in April of 1992, and the full season schedule was still played. The lockout at the start of the 1994-95 season reduced the schedule to 48 games and resulted in the collective bargaining agreement, which would eventually expire in 2004 and result in the loss of an entire season.

League commissioner Gary Bettman announced a lockout of the players union and ceased operations at the NHL head office when no new agreement could be formed. The league was shut down for a record 310 days, and the NHL became the first professional sports league to lose an entire season. In July 2005 a new collective bargaining agreement was formed and passed, allowing the NHL to resume operations for the 2005-06 season.

Unique Rules
Many rules observed by the NHL are the same as those observed by the International Ice Hockey Federation (such as three, 20-minute periods) but there are certain unique aspects that are worth noting.

During the regular season, if the game is tied after 60 minutes, a five-minute sudden-death overtime period is played four-on-four. If the game remains tied at the end of overtime the game enters a shootout, were three players for each team take a penalty shot. Still if no winner can be decided, single shooters from each team alternate until a winner can be determined.

During playoffs the overtime rules completely change. The shootout is completely eliminated and overtime periods are 20 minutes in length and played five-on-five. There is no theoretical limit to how long these games could go, though none have gone beyond six overtimes to this point.

Behind each goal there is a trapezoidal region in which the goalie can touch the puck. The area that is both outside this trapezoid and behind the goal line is off-limits for goalkeepers, and they will get a penalty for touching the puck in this area. This rule is often referred to as the "Brodeur rule" after goaltender Martin Brodeur, whose exceptional puckhandling skills is said to be the cause for the rule.

In the NHL, icing is only called when a defending player touches the puck before an attacking player, unlike the IIHF, where players stopped as soon as the puck crosses the goal line. Also, when a team is guilty of icing the puck they are not allowed to make a line change before the ensuing face-off.

The NHL also has differing penalty rules. A player may get a major penalty for a serious or dangerous infraction of the rules. The entire duration of a major penalty must be served even if the other team scores.

The NHL awards one point to a team losing in overtime or shootout, and is the only major sports league to award points for a loss.

Season Structure
An NHL season is divided into an exhibition season, the regular-season, and the postseason or Stanley Cup playoffs.

The regular-season follows a predetermined schedule of 82 games that goes until mid April. Each team plays 41 home games and 41 road games. Each team also plays six games against every team within their division and at least one game against every other team in the league. At the end of the regular season, the team with the most points overall is awarded the President's trophy and home ice advantage throughout the playoffs. In each conference, the top eight teams qualify for the playoffs. The three division winners in each conference are seeded one through three, with the non-division winners in the remaining five spots.

The Stanley Cup playoffs begins in April and lasts until the beginning of June. It consists of four, best-of-seven series with the winner being crowned Stanley Cup champion. In each round, the highest seed plays the lowest seed, the second-highest against second-lowest, and so forth. The higher seed always has home ice advantage, that is, four of the seven games are scheduled at their team's arena. The first, second, fifth, and seventh games are played at home for the higher seed. In the Stanley Cup Finals is played between the champions of each conference. The team with more points during the regular season gets home ice advantage, regardless of where they were seated within their own conference.

Trophies and Awards
Every year the NHL presents a number of trophies in addition to the Stanley Cup. The President's trophy is awarded to the team with the most points during the regular season. Most trophies are awarded to individual players either for their statistical accomplishments over the course of the season (or postseason) or as voted on by the Professional Hockey Writers' Association or team general managers. There are over 20 trophies and awards handed out each year, including:

Hart Memorial Trophy - league MVP (regular season)
Vezina Trophy - league top goaltender
Calder Memorial Trophy - most outstanding rookie player
Art Ross Trophy -player with the most points at the end of the regular season
Conn Smythe Trophy - playoff MVP