The World Series has been the annual championship series of the highest level of professional baseball in the United States and Canada since 1903, concluding the postseason of Major League Baseball. Since the Series takes place in October, sportswriters many years ago dubbed the event the Fall Classic, a usage reflected in the official symbols of the 2008 World Series; it is also sometimes known as the October Classic or simply The Series. The reigning World Series Champions are the Philadelphia Phillies.

The World Series is played between the champion clubs of the American League and the National League, which collectively include (today) 29 clubs based in the United States and one club from Canada, whose rosters contain players from around the world. With the exception of 1904 (boycott) and 1994 (player strike), the "modern" World Series has been played every year since 1903. Baseball has employed various championship formulas since the 1860s. When the term "World Series" is used by itself, it is usually understood to refer to the "modern" World Series exclusively.

The term "World Series" is derived from "World's Championship Series", a term which first appeared in the 1880s and continued into the early 1900s. This was eventually shortened to "World's Series" and then "World Series". While no international federation has ever sanctioned the series as a world championship event, its winners are still sometimes referred to informally as "world champions" by baseball players, owners and writers within the United States and Canada.