Sudden-death overtime has traditionally been used in playoff and championship games in hockey. It has been used in the National Hockey League throughout the league's history. The first NHL game with sudden-death overtime was game four of the 1919 Stanley Cup Finals. Currently, the NHL, American Hockey League, and ECHL also use the sudden-death system in their regular seasons, playing a five-minute overtime period when the score is tied at the end of regulation time.
In 2000, the AHL reduced the teams to four players each during the five-minute overtime. (But any two-man advantage is administered with five-on-three play rather than four-on-two.) The ECHL and NHL both changed to the four-on-four overtime format in 2001, with the International Olympic Committee following by no later than 2010.
If neither team scores during this period, the teams use a penalty-shot shootout, consisting of three players in the NHL or five players in the minor leagues (AHL, ECHL, UHL, Central), to determine the winner. In the NHL, if no team wins this shootout, a 1-by-1, sudden-death shootout ensues. No player may shoot twice until every non-goaltender on the bench has taken a shot.
During championship playoffs, however, all games are played to a conclusion resulting in a victory for one team and a loss for the other. These are true sudden-death games, which have gone on into as many as six additional full 20-minute periods with five players, instead of the five-minute period with four players.
International hockey uses a penalty-shot shootout for knockout rounds if neither team scores after one 20-minute, sudden-death overtime period. The shootout is decided round by round (in other words, if one team scores in the first round and the other does not, the game is over, unlike most professional leagues), and players can shoot as many times as the team desires. (There is no overtime in round-robin games.)