Major League Baseball (MLB) is the top professional baseball organization in the United States and Canada. MLB consists of 30 teams. The teams are divided into the American League (AL) and the National League (NL). Each league is divided into three divisions—East, Central, and West. MLB is the oldest major professional sports organization in the United States and Canada.
The MLB regular season lasts from late March or early April to late September or early October. The top six teams in each league qualify for the postseason playoffs, which may last as late as November. The finalists from each league play in the World Series, a best-of-seven-games series that determines the MLB champion. 
Beginnings.
Professional baseball in the United States dates back to 1869, when the Cincinnati Red Stockings (now the Reds) became the first officially all-professional baseball team. In 1871, the National Association was organized as the first professional baseball league. Major League Baseball officially dates back to the creation of the National League in 1876. Other, short-lived leagues were created soon after, including the American Association (1882), the Union Association (1884), the Players League (1890), and the Federal League (1914). The only other top league to flourish, however, was the American League, which began as a minor league and advanced to major league level in 1901. In 1903, the AL and NL were given equal status as baseball’s major leagues. That year, the AL and NL champions met in the first MLB World Series. All other leagues were given lower status, collectively known now as Minor League Baseball (MiLB).
Growing popularity.
MLB quickly gained popularity, and the World Series dominated national newspapers every autumn. Fans followed such star players as Ty Cobb, Walter Johnson, Christy Mathewson, Babe Ruth, and Honus Wagner—players who rivaled the popularity of movie stars and the president of the United States. Gambling on games became a problem, however. Probably the most famous example of the problem of gambling in baseball was the 1919 “Black Sox” scandal. This scandal involved members of the Chicago White Sox allegedly accepting bribes to intentionally lose the World Series. In 1920, MLB named a U.S. district judge from Illinois, Kenesaw Mountain Landis, as the first baseball commissioner, a position meant to help ensure the integrity and fairness of the game. 
MLB maintained its popularity through the Great Depression, a worldwide economic slump of the 1930’s. MLB ballparks offered affordable entertainment at a time when few people had extra money to spend. Fans could also follow the sport for the price of a newspaper or for free on the radio. Innovations of that era included the first All-Star Game in 1933, the first MLB night game in 1935, and the creation of the National Baseball Hall of Fame in 1936. Baseball suffered from down years during World War II (1939-1945) as many top players left the game for military service. United States President Franklin Delano Roosevelt insisted the game continue, however, saying, “it would be best for the country to keep baseball going.” Great MLB players who lost seasons to military service included Joe DiMaggio, Bob Feller, and Ted Williams.
Changes.
The National League was founded as a segregated league that did not allow Black players. This policy continued by unwritten agreement among the owners after the creation of the American League. (Successful separate professional Negro leagues had existed since 1920.) As American society evolved, however, so did baseball. In 1947, Jackie Robinson “broke the color barrier” by becoming the first Black player in modern Major League Baseball. More Black players soon joined the majors, and combined with new stars and the return of players from the military, a golden era of baseball boomed in the postwar years. Players such as Robinson, Hank Aaron, Ernie Banks, Mickey Mantle, and Willie Mays thrilled baseball fans in the 1950’s and 1960’s. MLB changed and expanded during this era as some teams moved west, new teams were added, more night games were played, and national television broadcasts added to the sport’s already immense popularity. Artificial turf, a popular innovation at the time, soon carpeted many new stadiums, and divisional playoffs were introduced in 1969. Prior to that year, the regular season champions of each league had gone directly to the World Series.
In 1975, MLB players won the right to free agency, meaning they could choose to move from team to team as their contracts expired. Previously, players had been the exclusive “property” of the ball club that originally signed them. This policy meant a player could change teams only by being traded or released—and the player had little say in it. This practice kept many players on one team for their entire careers, and it kept their salaries artificially low even as team owners’ profits soared. Free agency changed all that, ushering in an era of million dollar players who might play for several teams during their careers. Labor tensions between owners and players resulted in strike-shortened MLB seasons in 1972, 1981, and 1994. Great players of the 1970’s and 1980’s included Rod Carew, Steve Carlton, Reggie Jackson, Joe Morgan, and Mike Schmidt.
Issues and innovations since the 1990’s.
In the 1990’s, the problematic use of steroids (a catch-all term at the time for performance-enhancing drugs, or PED’s) changed MLB, as some players grew unnaturally stronger and many longstanding records were broken. Testing for PED’s began in 2002, and penalties for PED use gradually increased, greatly reducing the problem but not completely eliminating it. Star players such as Barry Bonds, Roger Clemens, Mark McGwire, and Sammy Sosa saw their outstanding careers tarnished by the suspected or proven use of PED’s. 
MLB introduced interleague play in 1997, creating regular season games between AL and NL teams. Prior to that year, AL and NL players saw each other only at the All-Star Game or the World Series. Other innovations since the 1990’s include revenue sharing (the transfer of money from high- to low-revenue teams), playoff expansion, and league and division realignment.
In March 2020, MLB spring training was suspended and the regular season was postponed because of the ongoing COVID-19 pandemic. COVID-19, a contagious respiratory disease, first broke out in China in late 2019. It soon spread throughout the world. Most other sports leagues and sporting events around the world were similarly suspended or canceled. In June 2020, MLB announced that a shortened, 60-game regular season was scheduled for 2020. The season began in July. Special precautions were put in place to protect players and others and to limit the spread of COVID-19. For example, games were played between regional opponents to limit travel.
Later in 2020, MLB announced that the Negro leagues will be raised to major league status. MLB planned to review the statistics and records of the approximately 3,400 players who played in the Negro leagues from 1920 to 1948 and incorporate them into major league historical records.
