Major League Baseball is the highest level of professional baseball in the United States and Canada. Specifically, Major League Baseball refers to the organization that operates the National League and the American League by a joint organizational structure that has developed gradually between them since 1901. In 2000, the two leagues ceased to be separate legal entities, with the commissioner's office assuming all responsibilities for running MLB. MLB effectively operates as a single league and as such it constitutes one of the major professional sports leagues of the United States and Canada. It is currently composed of 30 teams — twenty-nine in the United States and one in Canada. In conjunction with the International Baseball Federation, MLB also manages the World Baseball Classic.

Regular Season
The current MLB regular season is a 162-game schedule, organized typically 3-game series, with occasional 2- or 4-game series, and the rare 5-game series. Postponed games or continuations of suspended games can result in an ad hoc 1-game or 5-game series. A team's series are organized into homestands and road trips that group multiple series together. Over the course of a season, teams compete for one of the four playoff berths in their league. They can win one of these berths by either winning their division, or by capturing a wild card spot. In
many seasons, post-season teams are not determined until the very end of the season, while in other years, a post-season team can be decided as early as August.

All-Star Game
In early July — just after the midway point of the season — a three-day break is taken and the Major League Baseball All-Star Game is held. The All-Star game features a team of players from the National League (NL) — led by the manager of the previous NL World Series team — and a team of players from the American League (AL), similarly managed, in an exhibition game.

Post-season
When the regular season ends after the first Sunday in October (or the last Sunday in September), eight teams enter the post-season playoffs. Six teams are division champions; the remaining two "wild-card" spots are filled by the team in each league that has the best record but is not a division champion (best second-place team). Three rounds of series of games are played to determine the champion:
1. American League Division Series and National League Division Series, each a best-of-five-games series.
2. American League Championship Series and National League Championship Series, each a best-of-seven-games series played between the surviving teams from the ALDS and NLDS. The league champions are informally referred to as the American and National League pennant winners.
3. World Series, a best-of-seven-games series played between the champions of each league.

Within each league, the division winners are the #1, #2 and #3 seeds, based on win–loss records. The wild-card team is the fourth seed—regardless of its record—and is paired with the highest seed outside of its own division in the first round of the playoffs, while the remaining two division champions play each other.

Because each postseason series is split between the two teams' home fields, home-field advantage theoretically does not play a significant role unless the series goes to its maximum number of games, in which case the final game takes place on the field of the team holding the advantage.