in baseball, a postseason play-off series between champions of the two major professional baseball leagues of the United States: the American League (AL) and the National League (NL).

The World Series began in 1903 after the cessation of hostilities between the NL and the newly formed AL. Boston (AL) defeated Pittsburgh (NL) five games to three in a best-of-nine-game series. Attendance was just over 100,000, and the players' shares of receipts were slightly more than $1,000 each. In 1904 the New York Giants (NL) refused to play Boston, again the AL champion; but the series resumed in 1905 and continued annually until 1994, when a prolonged players' strike forced its cancellation that year. A seven-game format has been standard since 1922. Beginning in 1955, one player has been voted the Most Valuable Player of each series, a great honour in baseball. Montreal and Toronto were granted major league teams in 1969 and 1977 respectively—the first Canadian teams in major league baseball; Toronto's World Series win in 1992 was the first victory for a non-U.S. team. The New York Yankees of the AL have won the most series.

The World Series name has been applied to several baseball championships of lesser import, including the Junior World Series, played between champions of the International League and the American Association (both American professional minor leagues), and the Little League World Series, an annual event with international representation for teams of boys and girls 9 to 18 years old.

For details on the 2008 Major League Baseball play-offs, see 2008 World Series.