From BR Bullpen
The 20th century (1901-2000) was the first full century for professional baseball. While the rules had been primarily set in the 19th Century, many new elements were added to the baseball world throughout the decade.
 Rule Changes
The most notable rule changes might have been the elimination of new spitball pitchers after 1920 (with some legal spitballers allowed to continue from before that season) and the addition of the Designated Hitter in the American League in 1973 after some minor leagues had first adopted it in the 1960s. Free agency was established in the 1970s after much pressure from players; Marvin Miller and a revitalized MLBPA helped create this institution. While preventing players from being tied to one team that could dictate their salary, it also helped create a financial imbalance in baseball by which rich teams could once again dominate the scene.
While not a rule change per se, the construction of purpose-built stadiums with permanent stands was also instrumental in changing baseball. This led to a fixed size for the outfield (within certain parameters), better facilities for fans and players alike, and by the 1960's, the introduction of artificial turf and domed stadiums, which had their own particular effects on playing conditions. Equipment also evolved markedly, particularly gloves and protective gear for catchers and batters, significantly reducing the risk of injuries. One new piece of equipment, the aluminum bat, was introduced in many levels of baseball, but not in professional leagues in the USA.
At the beginning of the century, professional games were usually directed by one umpire. This evolved to the two and then four-umpire system which is now in place in Major League Baseball, although minor leagues will often use fewer umpires. Umpiring became much more professionalized, with the development of the first training schools in the late 1920's, and significant improvements in working conditions at the major league level following a series of bitter strikes by umpires between the late 1970s and 1990s.
 Postseason Play
One major introduction early in the decade was the World Series. While there had been a World's Series and Temple Cup in the 19th Century, neither was a long-lasting factor in the baseball world. With the rise of the American League to major league status, the World Series was begun in 1903 and played every year thereafter in the decade except for 1904 and 1994. In 1969, a second level of post-season play was added, the League Championship Series, and a third level, the Division Series, in 1994.
 International Baseball
Many international leagues were begun in this century, starting with Hoofdklasse Honkbal in 1922 in the Netherlands. Three years later, the Mexican League was formed. The Japanese Professional Baseball League was started in 1936. Serie A1 in Italy got its start in 1948. The Cuban National League was formed after the Fidel Castro-led revolution, replacing the old Cuban Winter League. The Chinese Professional Baseball League, Dominican League, Korea Baseball Organization, Taiwan Major League, Panamanian League, Nicaraguan League, Venezuelan League, Venezuelan Summer League, Dominican Summer League, Mexican Pacific League and Puerto Rican League all started up during the 20th Century.
International tournaments that began in this decade included the Caribbean Series (1949), Baseball World Cup (1938), Intercontinental Cup (1973), Haarlem Baseball Week (1961) and Pan-American Games (1951). Baseball was added to the Olympics as a demonstration sport a few times before becoming a medal game in 1992.
 Blacks and baseball
The decade started on a bad note for African-Americans and dark-skinned Cubans as Organized Baseball had barred black players by the end of the 19th Century, depriving them of their best outlet. As a result, numerous black teams were formed, with the number increasing exponentially in the first couple decades of the century. The Negro National League was then formed in 1920 by Rube Foster as the first organized all-black league in 33 years and the first one to be long-lasting. The rival Eastern Colored League was formed three years later and the Negro World Series began a year after that as the Negro Leagues began to look like a mirror of the white major leagues. Negro League and major league teams carried on off-season exhibitions until they were banned by MLB chief Kenesaw Mountain Landis when the white players lost too much for his taste. After that, many white MLB players continued to play black stars, but not in formal team vs. team competitions.
The Negro Leagues thrived over the next couple of decades, but began to take hits from high-spending Latin American leagues, with players going to the Dominican League in 1937 and Mexican League starting a short while later. By 1940-1941, perhaps half of the top African-American and dark-skinned Cuban players were active in Mexico, weakening the US-based Negro Leagues.
The big blow to the Negro Leagues, though, came in 1946, when Branch Rickey signed Jackie Robinson for the Montreal Royals, the Brooklyn Dodgers' top farm team, without compensating the Kansas City Monarchs, his former club. The move helped integrate Major League Baseball and former Negro Leaguers dominated the MLB scene for the next couple decades as Robinson, Monte Irvin, Larry Doby, Willie Mays, Ernie Banks, Hank Aaron, Roy Campanella and more became instant stars in the majors. The Negro Leagues stopped receiving much coverage even in black periodicals and they quickly faded away; by the 1950s, they were practically gone with only a few clowning teams and young prospects to show for note.
The presence of African-Americans in baseball continued to increase. On September 1, 1971, the Pittsburgh Pirates put out an all-black lineup. Over the next couple of decades, though, the number of black players began to decrease as the community focused its attention on other athletic venues. Baseball did continue to become increasingly integrated as the decade extended, though, with an influx of Latino players and the first Asian players.
 Women and Baseball
Highlights of the century for women in the baseball world cup included the strongest women's league to date (the All-American Girls Professional Baseball League) (1943-1954), the first woman to sign a pro baseball contract (Eleanor Engle), the first woman to play in the Negro Leagues (Toni Stone) and the first woman to play in the minor leagues (Ila Borders). The Colorado Silver Bullets briefly toured the country as an all-female team in the 1990s. Additionally, the movie A League of Their Own popularized the topic.
 College Baseball
The College World Series got its start and the number of college baseball programs increased throughout the course of the century. By the end of the century, college baseball had become big business, drawing major media coverage, although it was still a notch below the popularity of college football and basketball.
 Minor League Baseball
The decade started off on a good note for the minor leagues, with the number of leagues rising from 13 in 1901 to 52 in 1910. After that, many small regional leagues collapsed and World War I struck a further blow, leaving only one league to complete the 1918 season (the International League). Expansion began again and there were 44 leagues by 1941, when World War II began to cause leagues to fold - only 9 finished the 1943 season. A year after the war ended, 31 leagues started off and by 1949, 59 leagues were playing a full schedule. This was unsustainable though and the spread of television helped erode markets all over the place; by 1959, 21 leagues were left. The number would fall to 17 by 1980 before the number of leagues began to increase and the MLB expansion helped the size of those leagues expand with more affiliated teams. In the 1990s, the independent leagues were formed for players too old to be prospects or who might have been overlooked in the amateur draft, and while many of these struggled, a number became a well-established part of the baseball landscape, providing an alternative model to organized baseball.
The nature of the minors changed as well. They were largely independent in the early decades of the century, but the establishment of farm teams began to cut into this element in the 1930s. The farm system became a key way to establish dynasties for major league clubs, with a nice pipeline of young talent available. The strongest teams had much larger farm operations than smaller, less-competitive clubs. The need for such large networks was abolished by the start of the amateur draft in 1965, guaranteeing similar talent flows for all clubs. However, by that time, independent minor league teams had practically disappeared from organized baseball.
 Prominent Players
In the majors, prominent stars included Babe Ruth, Walter Johnson, Ty Cobb, Ted Williams, Stan Musial, Rickey Henderson, Christy Mathewson, Honus Wagner, Nap Lajoie, Rogers Hornsby, Lou Gehrig, Mike Piazza, Tris Speaker, Mickey Mantle, Lefty Grove, Willie Mays, Frank Robinson, Tom Seaver, Nolan Ryan and Mike Schmidt. The Hall of Fame was formed to honor baseball's greats and was soon followed by the Salon de la Fama, Japanese Baseball Hall of Fame and similar institutions in other countries.
In the Negro Leagues, stars included Smokey Joe Williams, Oscar Charleston, Pop Lloyd, Bullet Rogan, Willie Wells, Josh Gibson, Buck Leonard, Cool Papa Bell, Satchel Paige, Bill Foster, Biz Mackey and Mule Suttles.
Nippon Pro Baseball stars included Sadaharu Oh, Masaichi Kaneda, Shigeo Nagashima, Tetsuharu Kawakami, Victor Starffin, Kazuhisa Inao, Katsuya Nomura, Ichiro Suzuki, Kazuhiro Kiyohara, Leron Lee, Randy Bass, Boomer Wells, Hiromitsu Ochiai, Eiji Sawamura and Yutaka Fukumoto.
In the minors, stars included Hector Espino, Ike Boone, Nelson Barrera, Bill Thomas, Smead Jolley, Buzz Arlett, Nick Cullop, Joe Hauser, Ollie Carnegie, George Payne, Joe Martina, Andres Mora, Ken Guettler, Frank Shellenback, Spencer Harris and Jigger Statz.
 Controversies and Scandals
Several controversies hit baseball, most notably the Black Sox Scandal in which eight members of the 1919 White Sox were barred from baseball for their involvement in plans to throw the 1919 World Series. Pete Rose was banned from association with baseball in 1989 due to accusations of his having bet on baseball games; while he denied it for years, he later admitted his guilt. Other players involved in betting or game-fixing scandals included Hal Chase, Gene Paulette, Benny Kauff, Rube Benton, Jimmy O'Connell and Jesse Levan. In Japan, the Black Mist Scandal ended the careers of several players, including star Masaaki Ikenaga. Drug scandals hit baseball in the 1980s and 1990s; among those to be affected were Keith Hernandez, Lonnie Smith, Joaquín Andújar, Willie Aikens, Rod Scurry, Tim Raines Sr., Dave Parker, Vida Blue, the Pirate Parrot, Dick Davis, Dwight Gooden, Steve Howe and Darryl Strawberry.
 Other News