The years following the conclusion of World War II were marked by rising attendance, the growth of the minor leagues, and in 1947 the racial integration of the game (for more on the integration of baseball, see Blacks in baseball, below). This period also was marked by new efforts by players to obtain better pay and conditions of employment. A portent of things to come was the formation in 1946 of the American Baseball Guild. Although the guild failed in appeals to national and state labour relations boards, its very existence led to reforms before the 1947 season: a minimum major league salary of $5,000, no salary cuts during a season for a major league player moved to the minors, weekly spring-training expense money of $25, a 25 percent limit on annual salary cuts, and establishment of a players’ pension fund.

Landis’s successor as commissioner, Albert B. (“Happy”) Chandler (1945–51), assured the soundness of the pension fund in 1950 by signing a six-year contract for broadcasting World Series and All-Star games; the television portion alone amounted to $1 million a year, with a large proportion earmarked for the pension fund. Radio and television rights for regular-season games remained with each club. Later commissioners included Ford C. Frick (1951–65), William D. Eckert (1965–69), Bowie Kuhn (1969–84), Peter Ueberroth (1984–89), A. Bartlett Giamatti (1989), Fay Vincent (1989–92), and Allan H. (“Bud”) Selig.

Movement and expansion

The postwar boom was short-lived, however. America was going through tremendous changes. Millions were moving out of the cities and to the suburbs, and population centres in the South and West were growing. Americans had more time and money to enjoy themselves, which they did through vacationing and outdoor recreation. Moreover, the rapid growth of television preoccupied the country. Baseball was slow to adapt. Major league clubs were located only as far west as St. Louis and no farther south than Washington, D.C. Many of the ballparks had fallen into disrepair, were outdated, and were inconvenient for surburbanites driving in for a game. Despite exciting play on the field, attendance began to wane. The added revenue from radio and television broadcast rights could not offset the losses at the gate. The 1950s saw the first franchise changes since 1903. In 1953 the Braves, always overshadowed in New England by the Red Sox, moved from Boston to Milwaukee (in 1966 the franchise moved again, to Atlanta, Georgia), where they were offered a new stadium. The next year the St. Louis Browns, themselves overshadowed by the Cardinals, moved to Baltimore and became the Orioles. In 1955 the Philadelphia Athletics franchise was moved to Kansas City, Missouri (and in 1968 to Oakland, California). The impact of these moves was slight compared with the move of the Dodgers and Giants from New York City to California (the Dodgers to Los Angeles and the Giants to San Francisco) in 1958. Frustrated in his attempts to win city support for a new stadium, Dodger owner Walter O’Malley jumped at an offer to relocate the team to Los Angeles, which was then the third largest city in the country. O’Malley persuaded the Giants to move to San Francisco in order to maintain their rivalry and ease the travel burden on National League teams.

Despite the betrayal felt by fans in Brooklyn and Manhattan, the moves were a successful business decision for the clubs. The decade of franchise movement was followed by several rounds of expansion that lasted into the 1990s. Expansion began in 1961 when the Washington (D.C.) Senators were moved to Minneapolis–St. Paul and renamed the Twins, and a new franchise was granted to Washington (also named the Senators); however, it lasted only until 1971, when it was transferred to Dallas–Fort Worth and renamed the Texas Rangers. Another American League franchise was awarded to Los Angeles (later moved to Anaheim as the California Angels, now known as the Los Angeles Angels of Anaheim) in 1961, and in 1962 the National League also expanded to 10 teams with new franchises in New York City (the Mets) and Houston, Texas (the Colt .45s; after 1964, the Astros). The 154-game season had been expanded in the American League to 162 in 1961; the National League followed suit in 1962.

Along with this first round of expansion came an era of superb pitching that dominated the league for a generation. The earned run averages for pitchers during this era averaged 3.30, and the major league batting average fell as low as .238 in 1968. Several changes in the game were believed to account for the resurgence of pitching; the strike zone was expanded in 1963; managers explored more strategic uses of the relief pitchers; and new glove technology improved defensive play. At the same time, a new generation of large multipurpose stadiums came into use. These stadiums typically used artificial turf that was harder and faster than natural grass. As a result, new emphasis was placed on speed in the field and on the base paths. Fearing that the dominance of pitching was hurting fan interest in the game, the major league tried to improve hitting by lowering the mound and narrowing the strike zone in 1969. In hopes of further increasing offensive play, the American League introduced the designated hitter in 1973. The changes did increase offensive output, but pitching still dominated through much of the 1970s.

In 1969 new franchises were awarded to Montreal (the Expos, the first major league franchise outside the United States) and San Diego, California (the Padres), bringing the National League to 12 teams. In the American League in 1969, new franchises in Kansas City, Missouri (the Royals), and Seattle, Washington (the Pilots), brought that league to 12 teams, and both leagues were divided into Eastern and Western divisions.

Play-offs between division winners determined the league pennant winners, who then played in the World Series, which was extended into late October. California, which had had no major league baseball prior to 1958, had five teams by 1969. Of the new franchises, only Seattle failed outright and was moved to Milwaukee, where it became the Brewers (moved to the National League in a 1998 reorganization). A franchise was again granted to Seattle (the Mariners) and to Toronto (the Blue Jays), bringing the number of American League teams to 14 in 1977. In 1993 the National League also was brought to 14 with the addition of teams in Denver (the Colorado Rockies) and Miami (the Florida Marlins). In 1998 the Arizona Diamondbacks (located in Phoenix) joined the National League, and the Tampa Bay (Florida) Devil Rays (now known as the Tampa Bay Rays) began play in the American League.

In 1994 both leagues were reconfigured into East, Central, and West divisions. The play-off format was changed to include an additional round and a Wild Card (the team with the best record among the non-division-winning teams in each league). The play-offs were again expanded in 2012, when a second Wild Card was added to each league. ***Under the revised system, the two Wild Card teams play a one-game play-off, with the winner advancing to the best-of-five-games division series.

An explosion of offense occurred in the mid-1980s and after. In particular, home runs increased dramatically, reaching record-breaking numbers from 1985 to 1987 and again in the late 1990s. The reasons for the change from dominant pitching to hitting were not entirely clear. Many claimed the ball had been engineered to fly farther; others claimed that continual expansion had diluted the quality of pitching. The improved off-season conditioning (that now often included weight lifting) made players stronger and quicker with their bats. The 1990s also saw another generation of new ballparks, many of which featured small dimensions that were more to the liking of power hitters.

During the later half of the 20th century, expansion was perceived by baseball executives as both a source of added revenue for clubs (large entry fees were charged to new franchises) as well as a means of generating new interest in the game. In 2001, however, concerns over economically underperforming clubs prompted owners to announce plans to eliminate two teams (widely believed to be the Minnesota Twins and the since-relocated Montreal Expos). The plan was put on hold after the player’s union pursued legal action to prevent the move, and a 2002 Minnesota court order that forced the Twins to play out the lease at their home stadium effectively ended the talk of contraction for the foreseeable future.