Baseball personnel have cheated by deliberately violating or circumventing the game's rules to gain an unfair advantage against an opponent. Examples of cheating include doctoring the ball, doctoring bats, electronic sign stealing, and the use of performance-enhancing substances. Other actions, such as fielders attempting to mislead baserunners about the location of the ball, are considered gamesmanship and are not in violation of the rules.

Throughout the history of baseball, the rules have frequently changed as the game continues to evolve. A few common rules most professional leagues have in common is that four balls is a base on balls, three strikes is a strikeout, and three outs end a half-inning.