For origins of the game, see Rounders, a game originating in England and Ireland. Unlike many other sports, the Official Baseball Rules have remained largely static during the modern era of the game. Many baseball players, fans, and administrators view the rules and traditions of professional baseball as time-tested and nearly sacrosanct.

This was not the case during baseball's early days, particularly in the late 19th century, when rules changed almost yearly, often significantly. The game as we know it today really only began to take shape in the late 1880s, and even after that, many significant rule changes were made during the rest of that century.

In 1857, under the "Knickerbocker" rules which governed until 1872, the current 9-inning format was adopted, replacing the previous rule that the first team to score 21 runs won. The next year, called strikes were recognized, and a batter was out if a ball, fair or foul, was caught on the fly or after one bounce. In 1867, the batter was given the right to call for a high or low pitch.

The National League was formed in 1876. Its rules changed almost yearly for the next quarter century. In 1880, a batter was out if the catcher caught the third strike; otherwise the batter got four strikes. Before 1883, pitchers were required to deliver pitches with their hand below their hips; in that year, the rule was changed to allow shoulder-high deliveries. Until 1887, batters could call for either a high or low pitch, and the strike zone was either above or below the waist. In 1885, the rules changed, until 1893, to allow bats to be flat on one side; beginning in 1893, they had to be round. In 1887, the rules were changed so that batters could no longer call for a pitch; and the strike zone was defined as from the shoulders to the knees. During this period, the pitcher's mound was much closer to home plate, foul balls were not counted as strikes, batters got four strikes, and the number of "called balls" resulting in a walk—which initially included strikes and foul balls- went from 9 to 8 to 7 to 6 to 5 and, in 1889, to 4. In that same year, the number of strikes went from 4 to 3. In 1887, a rule was adopted for that year only counting walks as hits, which played havoc with statistics. In 1892, the 154 game schedule was adopted. In 1893, the pitcher's mound was moved from 50 to 60.5 feet from home plate. In 1894, foul bunts were made strikes, and the infield fly rule was adopted with one out. In 1895, foul tips were made strikes, but not foul balls. In 1898, the first modern balk rule was adopted, as well as the modern rule for recognizing stolen bases. In 1901, the infield fly rule was extended to apply when there were no outs.

Because of the frequent and often radical rule changes during this early period, the "modern era" is generally considered to have begun in 1901, when the American League was also formed.

Some significant rule changes continued in the first quarter of the 20th century, but were much less frequent. In 1903, the American League adopted the foul strike rule. In 1907, the sacrifice fly rule was adopted. In 1910, cork centers were added to balls. In 1925, the minimum distance for a home run was made 250 feet.

After that, the rules remained virtually static for decades. In 1961, the 162-game schedule was adopted. In 1969, the pitcher's mound was dropped five inches and the strike zone reduced: from the armpits to the top of the knees. In 1973, the American League adopted the designated hitter rule—rejected to this day by the National League. This was probably the most controversial rule change in baseball's history, and is still subject to lively debate. Also controversial when adopted, although more generally accepted now, was the later introduction of inter-league play. The most recent significant rule changes, banning the use of steroids and other performance-enhancing substances, have had widespread support as protecting the integrity of the game. The use of steroids by many players over the last two decades has called into question a number of baseball records for both hitting and pitching.

The generally static nature of baseball's rules in the modern era has been a source of the sport's appeal to many fans, as it allows reasonable if imperfect comparisons to be drawn between players and teams of different eras. Such comparisons are not possible in sports in which the rules have changed significantly over the years. The generally static nature of the rules in the modern era also allows a modern fan to easily follow an account of a game played long ago. As a result, baseball arguably has more of a "history" than most other sports.