Baseball's origins are murky at best, and there is a lot of mythology behind it. But the game does bear similarities to cricket, which, like baseball, appears to have evolved from earlier bat-and-ball games popular in England and Western Europe. For decades, the invention of the game was attributed to Abner Doubleday, with the first game said to have occurred in Cooperstown, New York (which remains to this day the home of the Major League Baseball Hall of Fame), but historians now agree that neither had anything do with the game's origins. Modern baseball's actual roots seem to be in several similar bat-and-ball games that were played throughout the United States in the 19th century, with a version from the New York City area becoming popularized and spread across the country during and after the Civil War.

The game is played between two teams, each with nine players active at a time. The teams take turns playing defense and offense, with the same players playing both roles. While on offense, players come to home plate in order and try to hit a thrown baseball with a wooden or aluminum bat, with the objective of sending it far enough away that the batter can reach base before the ball can be relayed there. On defense, each of the nine players has a set position on the field: one pitcher, who throws the ball to the batter; one catcher, who catches any pitched balls that don't get hit; one baseman at or near each of the three bases; a shortstop between second base and third base; and three outfielders who roam the large expanse between the bases and the far edge of the playing field, trying to catch batted balls and relay them inward. Most leagues around the world also include a designated hitter (DH), a player who only bats, generally replacing the pitcher in the offensive lineup. As of 2022, the only significant league that does not use the DH is the Central League, one of the two leagues within Nippon Professional Baseball in Japan (the other NPB league, the Pacific League, uses the DH). From 1973 to 2021 (except in 2020), Major League Baseball used the DH only in the American League, but the National League started using it for good in 2022. Every defensive player wears a large leather glove (or, for catchers and first basemen, a mitt) on their non-throwing hand to aid in catching and fielding the ball. There is also a small group of umpires on the field who enforce the rules and make judgement calls on plays.

A batter has three chances, or strikes, to hit a ball into play; if the batter misses three times, he is out. The batter is also out if a fair ball is caught before touching the ground by anyone from the fielding team, known as a popup within or near the infield; a fly out if it's hit high in the air to the outfield; or a line out if it's hit on a line (also called a line drive). If a ball is hit on the ground but is then relayed to a base before the batter can reach it, that is called a ground out at first base and a force out on bases other than first, and if a runner is touched with the ball by the opposing team while running between bases, they are tagged out. If the fielding team gets two players from the batting team out in a single play, this is known as a double play; the much rarer triple play requires the fielding team to get three players from the batting team out in one play. One unique feature of fly outs or line outs that are hit sufficiently deep into the outfield with one or more runners on base is that those runners can tag up, meaning they can try to advance to the next base, including home plate, at their own risk, after they touch (tag) the base they are departing from when the ball is caught. A fly ball that scores a run in this way is called a sacrifice fly.

The pitcher must give the batter a reasonable chance to hit the ball; if the ball is thrown outside of the strike zone (which is over the plate, between the batter's knees and armpits) and the batter doesn't swing at it, the pitcher is charged with a ball. If the batter swings at the ball and misses, it is a strike regardless of where the ball is thrown. Four balls to the same batter allow the batter to advance to first base automatically, called a walk. Prolific hitters are sometimes intentionally walked to avoid the chance of them getting a run-scoring hit. In the event of a walk, runners may only advance a base if they are required to vacate their base in order for the batter to be able to take first base. A walk with all three bases occupied results in a run being scored. Another less usual reason for a batter to reach base is when they are hit by a pitch.

Once a batter reaches base, they become a runner, and can attempt to advance to the next base if the ball is still far enough away. Wherever the runner stops, they wait at that base as the next batter comes up. The goal of the runner is to make it around the bases and back to home plate. The runner can try to advance to the next base at any time; normally this occurs when the batter puts the ball into play, but if the runner advances on a pitch that wasn't batted into play it's called a stolen base (steal for short). The batter may also attempt to "steal" first base if the catcher misses or drops the ball, though this is only possible (or indeed even allowed) if said miscue took place on the batter's third strike. (Advancing to first base in such a situation is not scored as a stolen base.) If the runner is not forced to advance to a given base by runners behind him (which is always the case during a steal attempt), it's not enough for the defense to get the ball to the base before the runner; in order to record an out, the defense must tag the runner before he reaches the base. If the runner is successfully tagged out while attempting a steal, this is known as being caught stealing. Pitchers will try to keep runners close to first base (and, much less often, second or third base) to prevent a steal by periodically throwing to that base instead of home plate. Much less often, the catcher may throw to first if he or she sees a runner off that base. When a runner is caught off base and tagged out in this situation, it's known as a pickoff and scored as caught stealing. If a runner reaches home plate safely, a run is scored, and the team with the most runs at the end of the game wins. A team keeps batting until three outs have been recorded, so they can score unlimited runs during their turn at bat. Once the third out is made, the teams switch sides. A full pair of these turns is called an inning, and the game lasts for nine innings (less at the amateur levels) unless the score is tied at the end. In the event of a tie, extra innings are played until an inning ends with one team ahead. If a team is ahead after the half of the ninth inning in which the opposing side would have had the opportunity to score, the second half of the ninth inning is not played as all it could do is increase the margin of victory, which is — as per usual in American sports — hardly ever relevant.

A ball hit out of play beyond the foul lines is a foul ball and counts as a strike (unless the batter already has two strikes). If the batted ball hits the batter while in the batter's box, it is also considered foul regardless of the batter's actual position at that time. However, the foul lines themselves are in play, as are the foul poles, vertical extensions of the foul line placed on each side of the outfield fence or wall. A batted ball that bounces in fair territory and passes first or third base in fair territory is fair regardless of where it eventually lands, as is a ball that hits first or third base, or a ball which first bounces on a foul line after passing first or third base. If a foul ball that is not hit high in the air is caught by the catcher before hitting the ground or touching another player, this is known as a foul tip and also counts as a strike, but unlike a regular foul ball, may also count as a third strike. If the foul ball is caught by any other fielder before touching the ground, or if it is hit high in the air and caught, it counts as a regular fly out, and is often called a foul out. If the batter hits the ball beyond the outfield wall within the foul lines without touching the ground (or hits a foul pole on the fly), it's an automatic home run and the batter and all runners may freely advance to home plate. A home run, or "homer", is always sure to get the fans on their feet; it's the game's signature play. A home run that is hit with all three bases occupied (hence scoring four runs) is known as a grand slam, though this is quite rare. An even rarer event is an inside-the-park homer, in which the batter completes a trip around the bases while the ball is in the field of play.

If a hit ball bounces off the ground on its way beyond the outfield wall within the foul lines, this is an automatic double, also known (technically incorrectly) as a ground rule double. This results in the batter and all runners automatically advancing two bases (hence scoring runs for the runners on second and third base). Technically, a "ground rule double" is a double awarded when a batted fair ball is affected by the features of a given ballpark. Examples of actual ground rule doubles include balls getting caught in the ivy covering the walls of the Chicago Cubs' home of Wrigley Field, and balls hitting some of the catwalks above the Tampa Bay Rays' home of Tropicana Field.

Some levels of play have a run rule, known in the international rule set as the "run-ahead rule" and informally as the mercy rule. In international play (i.e., games involving national teams), a game ends if a team has a lead of 10 or more runs after the trailing team has completed at least seven innings of a nine-inning game. In seven-inning games (used in women's international play and in international doubleheaders), the run rule can be invoked after five innings, with the same 10-run lead required. Youth baseball invariably uses a run rule; for example, in the flagship 12-and-under division of Little League Baseball, in which games are only six innings, the rule is invoked if the lead is at least 15 runs after the trailing team has completed three innings, or 10 runs after four innings. However, US college baseball almost never uses the run rule, and professional baseball leagues do not use it at all.

Beyond these basics, there are many quirks and special cases in the baseball rules, some of which baffle even die-hard fans to this day. Some more or less new rules as of 2023 that may confuse even seasoned baseball fans include the rule that each half inning of an extra-inning game in the Major Leagues starts with a runner on second base, and also the fact that the manager of each team may challenge a certain number of play calls per game. When such challenges occur, a time out is taken by the umpires, and an official reviews replays to determine, for example, whether a ball hit over the fence was foul or a home run or whether a runner was really safe or out on a base. There is much more to say about the rules of baseball and how the game is played, but part of the fun of going to games is speaking with other fans who are happy to explain the rules of the game and the roles of different kinds of players to people new to baseball, so while the explanations in this section should help you understand many of the basics, it is not at all bad to leave some of the details for when you are at the game.

Canada is the only foreign county represented in the United States' Major League Baseball.

The Toronto Blue Jays have played in the American League since 1977, and have been the only team outside the U.S. to appear in and win a World Series with victories in 1992 and 1993. The Montreal Expos played in the National League from 1969 until 2004, and retain a nostalgic popularity in the city.

The Vancouver Canadians are the only Minor League Baseball team based in Canada. They play in the Northwest League.

• The Canadian Baseball Hall of Fame in St. Marys, Ontario, is dedicated to preserving Canada's baseball heritage. There are thousands of artifacts on display in the museum.
• The Saskatchewan Baseball Hall of Fame in Battleford, Saskatchewan, has a display of memorabilia including uniforms, photographs, baseball bats, baseballs, equipment, trophies, and "Canada's Biggest Bat".

Baseball (野球 yakyū) has been hugely popular since it was introduced to Japan in the 1870s by an American professor. Baseball fans travelling internationally may find Japan to be one of the great examples of baseball popularity outside of the United States. Baseball is not only played in many high schools and by professionals, but also referenced much in Japanese pop culture as well.

In addition, many Japanese players have gone on to become top players in Major League Baseball, such as Ichiro Suzuki and Shohei Ohtani. The official Japanese baseball league is known as Nippon Professional Baseball, or simply known as Puro Yakyū (プロ野球), meaning Professional Baseball, and it is regarded by many to be the strongest professional baseball league outside of the United States. There are also some local independent leagues in Japan.

The Japanese national baseball team is also considered to be one of the strongest in the world, having won the inaugural World Baseball Classic in 2006, as well as the second edition in 2009 and the most recent in 2023.

Tickets to baseball games are generally easy to get, even on the day of a game, although popular games should of course be reserved in advance. Tickets start around ¥2,000. If you are interested, be sure to leave 4–5 hours free. At some stadiums, you can bring in outside food and drink, which is a good way to save some money instead of paying prices inside the stadium (¥800 for a pint of beer); you'll just need to have your bag inspected and pour your drinks into disposable cups. Especially in Osaka, it's also popular to visit local restaurants or bars where the entire store will be taken over by fans loudly singing, chanting, and cheering non-stop through the entire game. The rules in Japanese baseball are not much different from baseball in United States, although there are some minor variations. The biggest rivalry is between Tokyo's Yomiuri Giants (a national favorite and top performer, although equally disliked by many) and Nishinomiya's Hanshin Tigers (a perennial underdog, widely known for having the most boisterous and dedicated fans, along with many cheers, songs, and traditions).

Japan has two national high school tournaments each year that draw possibly more attention than the professional game. Both are held at Kōshien Stadium, a stadium in Nishinomiya City near Kobe that seats over 50,000 and also hosts NPB's Hanshin Tigers.

• The National High School Baseball Invitational Tournament, commonly known as Spring Kōshien (春の甲子園 haru no kōshien, or センバツ senbatsu) – Held in March, featuring 32 teams invited from throughout the country.
• The National High School Baseball Championship, commonly known as Summer Kōshien (夏の甲子園 natsu no kōshien) – A two-week event in August, it is the final phase of a nationwide tournament structure. 49 teams participate in the final phase—one from each of Japan's prefectures, with second teams from Hokkaido and Tokyo.

Baseball is the second biggest team sport in Mexico after football (soccer) with two top-tier professional leagues, Liga Mexicana de Beisbol (LMB) and the Liga Pacifica (LMP). The best Mexican league players make it to Major League Baseball teams in the U.S., but that is not a universal goal in Mexico, where many players enjoy long careers (particularly in the Mexican League teams) and are relatively well paid. A considerable number of American players from MLB teams have played in the LMB or have found coaching positions there.

The Liga Mexicana is the most prominent league. It began in 1925. It is organized as two divisions, North and South, with 20 teams playing a 114 game season, from April to September. Teams are located in many popular tourist destinations including Cancun (Tigres), Mexico City (Diablos Rojos), Guadalajara (Mariachis), Monterrey (Sultanes), and Tijuana (Toros).

The Pacific League (LMP) is a winter league with 10 teams, mostly in the states of Sinaloa and Sonora, but with teams in Mexicali (Aguilas), Guadalajara (Charros), and Monterrey (Sultanes). The champion of the Liga Pacifica goes on to compete in the Caribbean Series against professional champions from other Latin American countries.

LMB has a minor league called the Liga Norte de Mexico which serves as a feeder league, providing developed players to LMB teams. Liga Norte consists of 7 teams in northwestern Mexico (Baja California, Baja California Sur, and Sonora).

Monterrey and Guadalajara are the two cities with teams in both the Liga Mexicana and Liga Pacifica, so you can catch a game there at almost any time of the year. The Mexican Baseball Hall of Fame is in Parque Fundidora in the northern city of Monterrey.

• Serie del Rey - the LMB championship is a best-of-seven series pitting the North Division champion team against the South Division champion team. The winning team goes home with the Copa de Zaachila. The Serie del Rey has been played annually since 1925.

Baseball was brought to Korea by American missionaries in 1904 and is the most popular sport in the country. Most cities have a team, the biggest are sponsored by the largest South Korean companies, and many South Korean players have become famous Major League Baseball players in the United States. The top baseball league in South Korea is the KBO League, featuring 10 teams from 8 cities. The South Korean national baseball team is also regarded as one of the strongest in the world, and finished second at the 2009 World Baseball Classic.

In 2024, it achieved 10 million viewers for the first time.

Baseball was brought to Taiwan by the Japanese during the colonial period. Its popularity rose greatly when the Taiwanese baseball team finished second in the Japanese national championships in 1931. Today, baseball retains a strong following and remains by far the most popular spectator sport in Taiwan. Several Taiwanese players have also gone on to successful careers in the U.S. Major League Baseball and Japanese Nippon Professional Baseball, and the Taiwanese national team is considered to be one of the strongest in the world. The top baseball league in Taiwan is the Chinese Professional Baseball League (CPBL), which features six teams. CPBL teams are sponsored by and named after large Taiwanese and Japanese corporations.

Main article: Baseball in the United States

In almost any place in America, there will be a baseball game going on during the spring and summer months, and watching it is a fantastic way to meet the locals and experience American sports culture. The main professional baseball league in the United States is Major League Baseball (MLB), and there also numerous semi-professional and amateur leagues that are much more accessible, plus college and high school games.