Initially brought on by the threat of a proposed Continental League, expansion has occurred six times in Major League Baseball the 20th Century. In 1961, the American League added the Los Angeles Angels and Washington Senators (which replaced the previous club of that name that became the Minnesota Twins). The New York Mets and Houston Colt .45's (later the Astros) joined the National League in 1962. For the 1969 season, the majors expanded by four teams. They were the Kansas City Royals, Montreal Expos, San Diego Padres, and Seattle Pilots (who became the Milwaukee Brewers after one season). In 1977, the Seattle Mariners and Toronto Blue Jays joined the American League. The National League added two clubs, the Colorado Rockies and the Florida Marlins, in 1993. The most recent expansion was in 1998, when the Arizona Diamondbacks and Tampa Bay Devil Rays began play.
The rosters of expansion teams are generally stocked through an expansion draft of players from existing teams. Specific procedures for the draft have varied over the years, but expansion clubs usually end up stocked with has-beens, journeymen, and young players.
Generally, expansion teams have not been competitive in their first several seasons. The Mets, who set a record for futility when they won only 40 games in their first year, finished in ninth or tenth place in the National League every season before finally winning the World Series in 1969. The Mariners were a sub-.500 club for their first 14 seasons. The more recent expansion clubs have fared a lot better. The Marlins won the World Series in their fifth season, and the Diamondbacks accomplished the feat in their fourth.
In an interesting about-face, Major League Baseball considered the contraction of two teams (the Montreal Expos and Minnesota Twins) following the 2001 season. This did not work out - thankfully. Talk of expansion began anew in the early 2010s, as labor peace and growing revenues meant that there were no teams struggling financially anymore, while it was proving awkward to devise a schedule with 30 teams. Among the leading candidates for a new expansion were Montreal, QC, where there had been a significant rebirth of interest for major league baseball a decade after the Expos' relocation to Washington, DC, Charlotte, NC, and Mexico City. However, Commissioner Rob Manfred indicated at a press conference around the 2017 All-Star Game that before he would seriously consider expansion, he wanted to resolve outstanding ballpark problems tat were hampering two franchises, the Oakland Athletics and Tampa Bay Rays.
The "expansion era" refers to the Major League Baseball seasons played since 1961 (or sometimes since 1962), as the changes brought by expansion, such as the lengthening of the schedule, and a few years later the addition of new postseason rounds, are considered to have significantly altered overall playing conditions. It also coincides with other trends that started a few years earlier, such as the break-up of New York City's stranglehold on the World Series, the appearance of video records of many games and of detailed play-by-play records, the reform of the minor leagues, the changed demographics of ballplayers, etc., that all contribute to making the years around 1960 a major turning point in baseball history.
Other leagues, including minor leagues, foreign leagues, winter leagues and independent leagues, have also undertake expansion processes to add new teams.
Levels of Expansion
- Expansion of 1961 - American League expands to 10 teams
- Expansion of 1962 - National League expands to 10 teams
- Expansion of 1969 - Both leagues expand to 12 teams
- Expansion of 1977 - American League expands to 14 teams
- Expansion of 1993 - National League expands to 14 teams
- Expansion of 1998 - Major League Baseball expands to 30 teams
- Frank P. Jozsa, Jr.: Major League Baseball Expansions and Relocations: A History, 1876-2008, McFarland, Jefferson, NC, 2010.
- Maxwell Kates and Bill Nowlin: Time for Expansion Baseball, SABR, Phoenix, AZ, 2018. ISBN 978-1-943816-89-7
- Andy McCue: Stumbling around the Bases: The American League’s Mismanagement in the Expansion Eras, University of Nebraska Press, Lincoln, NE, 2022. ISBN 978-1-4962-0703-6
- Manny Randhawa: "A look back at MLB Expansion Draft history", mlb.com, December 13, 2021. 
- Frank Zimniuch: Baseball's New Frontier: A History of Expansion, 1961-1998, University of Nebraska Press, Lincoln, NE, 2013. ISBN 978-0803239944