In Major League Baseball, the 1970s were dominated by a few teams. Six teams accounted for 18 of the 20 participants in the World Series during the decade, and three of them - the Oakland Athletics from 1972 to 1974, the Cincinnati Reds in 1975-1976 and the New York Yankees in 1977-1978 - won consecutive World Series. The most publicized events of the decade were Hank Aaron breaking Babe Ruth's record of 714 career home runs early in the 1974 season, and Pete Rose's 44-game hitting streak in 1978. That year's pennant race, which saw the New York Yankees overcome a seemingly insurmountable lead by the Boston Red Sox, culminating in a one-game playoff won by the Yankees thanks to an unlikely home run by shortstop Bucky Dent, is the most famous of the era. In addition, the 1975 World Series pitting the Cincinnati Reds and the Boston Red Sox is considered by many to have been the most exciting ever played.
The decade began with two franchise movements, with the Seattle Pilots becoming the Milwaukee Brewers in 1970, and the Washington Senators becoming the Texas Rangers after the 1971 season. The expansion of 1977 added two more teams to the American League, the Seattle Mariners and Toronto Blue Jays. All of these changes affected the American League, as the National League would maintain the same alignment all decade, and indeed until the expansion of 1993. Stability was on its way, but it was not apparent yet, as in addition to the moves that did happen, the San Diego Padres almost moved to Washington, DC in 1974, the San Francisco Giants considered moving to Toronto, ON after the 1975 season, and the Oakland Athletics were close to finding a new home in Denver, CO for 1978.
On the field, the decade was characterized by remarkable parity between hitting and pitching, with a strong gain in popularity by the stolen base, that would culminate in the first years of the 1980s. Many teams developped a strong identity during the decade, based on their style of play: the Big Red Machine (Cincinnati Reds) and Pittsburgh Lumber Company (Pittsburgh Pirates) were known for pounding their opponents into submission in spite of indifferent starting pitchers (both teams had very strong bullpens, though), the Oakland Athletics were known for speed and pitching, while the Baltimore Orioles and Los Angeles Dodgers were known for dominant starting pitchers, home run hitting, and an excellent grasp of fundamentals drilled throughout the organization. Most of these teams were built thanks to strong farm systems and a clear organizational philosophy embodied in a long-serving manager like Sparky Anderson, Earl Weaver, Walter Alston or Danny Murtaugh, as were other strong teams of the latter half of the decade like the Kansas City Royals and Philadelphia Phillies. In contrast, the New York Yankees became great by following a different model, relying on signing free agents and trading for established players and seemingly changing their manager every year, a business model that would become increasingly common in years to come.
As the decade wore on, a number of trends began to become more apparent. Franchise stability was one, as was the introduction of a new type of stadium. Dubbed the cookie-cutter stadiums, they were multi-functional facilities built near large parking lots away from the center of their cities. Because of the need to host both baseball and football games, they usually had artificial turf and lacked the distinct character of established baseball shrines like Yankee Stadium, Fenway Park or Wrigley Field. Looking at the field, it was impossible to know if the game was being played in Busch Stadium in St. Louis, Riverfront Stadium in Cincinnati or Three Rivers Stadium in Pittsburgh. More futuristic ballparks, like Seattle's Kingdome and Montreal's Stade Olympique were more distinctive, but all lacked the warmth of traditional stadiums. In time, they would come to be seen as cold and impersonal, but in the 1970s such parks embodied modernity and the future of professional sports, with their plastic grass, flashing scoreboards, and loud public address systems. They were also conducive to the running game, to hitting doubles, and to high batting averages; speed became a more important part of the game than it had been at any time since the Deadball era of the 1900s and 1910s. Plastic not only overtook grass, it also became a part of uniforms, as many teams switched to bright-colored uniforms made of double-knit polyester. The major fashion victims of the era were the Pirates, who sported a seemingly endless variety of randomly garish combinations of gold, black and white, the San Diego Padres whose uniforms, in an early example of corporate convergence, reproduced the colors of McDonald's fast food restaurants, the property of team owner Ray Kroc, and the Houston Astros whose uniforms with rainbow stripes of different shades of red, orange and yellow failed to set a trend. Another bizarre landmark occurred in 1976, when the Chicago White Sox wore dark bermuda shorts for part of the season.
Two other events had a major effect on the game: in 1973, the American League introduced the designated hitter in an effort to boost hitting, and after the 1975 season, a landmark decision by arbitrator Peter Seitz in the case of pitchers Andy Messersmith and Dave McNally led to free agency becoming a reality for players. Labor strife was prevalent throughout the decade, as salaries rose significantly and many owners fought a rear-guard action to prevent a full opening of the floodgates: the start of the 1972 season was delayed by a strike and spring training was shortened in 1976, but the real confrontation would come in the 1980s. Similarly, umpires began agitating for better pay and working conditions, and went on strike at the beginning of the 1979 season, leading Major League Baseball to use amateur umpires and strike-breakers in order to play its games.
The minor leagues bottomed out in size in the 1970s, as the Northern League folded after the 1971 season and the Carolina League barely survived after being reduced to four teams in 1975 and 1976. The Lone Star League, a new minor league, part of organized baseball but composed of unaffliated teams, folded after two seasons in 1977, while the Inter-American League only lasted a few weeks in 1979 before collapsing. Attendance was poor throughout minor league baseball, franchises moved very frequently, and teams played in dilapidated or makeshift ballparks. It would take another decade for the renaissance to begin.
International baseball became more prominent in the 1970s. Talented American players continued to leave MLB to play in Japan, notably Willie Davis, Charlie Manuel and Davey Johnson. But for the first time a Japanese player also made a strong impression in America, Sadaharu Oh whose home-run hitting prowess drew comparisons with Aaron's. The Yomiuri Giants ended their run of 9 straight Japan Series titles, allowing other teams to win titles in the country. In 1973, the first Intercontinental Cup was held, and Cuban teams dominated amateur competitions, as Cuban players were barred from playing in the USA. Still, the number of Latin players in MLB grew significantly, with the small city of San Pedro de Macoris in the Dominican Republic becoming a household word among baseball fans for its ability to develop Major League talent.
- Dan Epstein: Big Hair and Plastic Grass: A Funky Ride Through Baseball and America in the Swinging '70s, Thomas Dunne Books, Macmillan, 2010. ISBN 0312607547
- Daniel A. Gilbert: Expanding the Strike Zone: Baseball in the Age of Free Agency, University of Massachusetts Press, Amherst, MA, 2013. ISBN 978-1-55849-997-3
- Paul Hensler: The American League in Transition, 1965-1975: How Competition Thrived When the Yankees Didn't, McFarland, Jefferson, NC, 2013. ISBN 978-0-7864-4626-1
- Paul Hensler: Gathering Crowds: Catching Baseball Fever in the New Era of Free Agency, Rowman & Littlefield, Lanham, MD, 2021. ISBN 978-1538132005
- Bill James: "The 1970s", in The New Bill James Historical Baseball Abstract, The Free Press, New York, NY, 2001, pp. 276-295.
- 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
- Josh Wilker: Cardboard Gods: An All-American Tale Told Through Baseball Cards, Seven Footer Press, New York, NY, 2010. ISBN 1934734160
- Frank Zimniuch: Baseball's New Frontier: A History of Expansion, 1961-1998, University of Nebraska Press, Lincoln, NE, 2013. ISBN 978-0803239944
-  Article examining the depressed power statistics of star hitters in the 1970s in The Hardball Times, part 1.
-  Article examining the depressed power statistics of star hitters in the 1970s in The Hardball Times, part 2.