In baseball, a pitcher can commit a number of illegal motions or actions that constitute a balk. Most of these violations involve a pitcher pretending to pitch when he has no intention of doing so. In games played under the Official Baseball Rules that govern professional play in the United States and Canada, a balk results in a dead ball or delayed dead ball. In certain other circumstances, a balk may be wholly or partially disregarded. Under other rule sets, notably in the United States under the National Federation of High Schools Baseball Rules, a balk results in an immediate dead ball. In the event a balk is enforced, the pitch is generally (but not always) nullified, each runner is awarded one base, and the batter (generally) remains at bat, and with the previous count. The balk rule in Major League Baseball was introduced in 1898.
A pitcher is restricted to a certain set of motions and one of two basic pitching positions before and during a pitch. If these regulations are violated with one or more runners on base, an umpire may call a balk. The batter at home plate does not advance on a balk.
Balk rules under other rule sets vary.
The pitcher's acts of spitting on the ball, defacing or altering the ball, rubbing the ball on the clothing or body, or applying a foreign substance to the ball are not balks; however, it will result in the pitcher's ejection from the game if caught.
A pitcher was allowed to feint toward third (or second) base, and then turn and throw or feint to first base if his pivot foot disengages the rubber after his initial feint. This is called the "fake to third, throw to first" play. However, Major League Baseball classified this as a balk beginning with the 2013 season.
If no runners are on base and the pitcher commits an otherwise balkable action, there generally is no penalty. However, delivering a quick return or pitching while off the rubber (which constitute balks when runners are on base) results in a ball being called with the bases empty. If the pitcher should commit an act confusing to the batter with nobody on, or if he stops his delivery or otherwise violates because the batter steps out or otherwise acts confusingly, time is called and the play restarted without penalty (whether or not runners are on base). If a pitcher repeatedly commits illegal actions without runners on base, he may be subject to ejection for persistently violating the rules.
If, during an attempt to execute the "hidden ball trick" (where the defensive team deceives the runner(s) as to the ball's location while the play is live), the pitcher stands on the rubber prior to the fielder revealing the ball and applying the tag, the runner is not out. Instead, it is a balk, with all runners on base being awarded their next base.
"Catcher's balk" is not a term in the official rules, but is sometimes used to describe the infrequent situation when the catcher is not completely within the catcher's box when the pitcher releases the ball in his delivery during an intentional walk. The balk is still charged to the pitcher, because such a pitch is defined as a "Pitcher Illegal Action."
A pitcher is not required to step off the rubber before throwing to an occupied base in a pick-off attempt. With his foot on the rubber in either the windup position or the "set" position, the pitcher may either: 1) deliver the ball to the batter: 2) throw to a base for a pickoff; or 3) step off the rubber.
MLB rules state that: "Pitchers shall take signs from the catcher while in contact with the pitcher's plate" (the rubber), but the rules do not describe the infraction as a balk.
The major league record for the most balks in one game is held by Bob Shaw, who had five balks in a May 4, 1963, game while pitching for the Milwaukee Braves against the Chicago Cubs. Four of the five balks came when the Cubs' Billy Williams was on base: one in the first inning, then three more in the third inning. In the latter frame, Shaw walked Williams, and then proceeded to balk him to second, third and home. Shaw's balks were blamed on his difficulty adjusting to a then-new point of emphasis in the rules: umpires were told to enforce the section of the balk rule strictly that required the pitcher, when going from the stretch to the set position, to come to a complete stop with his hands together for one full second before pitching. The rule had been virtually ignored before.
Knuckleballer Charlie Hough was once called for nine balks in a single major league exhibition game, occurring in March 1988. Hough was called for seven balks in a single inning of the game, as umpires set out to "enforce a full set position" for the coming season.
During the 1947 World Series (New York Yankees vs. Brooklyn Dodgers) Spec Shea dropped the ball trying to pick off Jackie Robinson at first base; after at least one other attempt, he dropped the ball and umpire Babe Pinelli waved Robinson to second base. 
A famous balk came in the first All-Star Game of 1961, when strong winds at Candlestick Park caused pitcher Stu Miller to sway erratically and be called for a balk. This story is often exaggerated in re-tellings of baseball lore, some having Miller being blown off the pitching mound.
The Los Angeles Dodgers defeated the Texas Rangers on June 18, 2015, when Rangers relief pitcher Keone Kela committed a balk in the bottom of the ninth inning of a tie game with Dodger Enrique Hernández at third base. There have been at least 21 such walk-off balks (or "balk-offs") in major league history since 1914.
On June 14, 2019, Dodgers closer Kenley Jansen intentionally balked during a game with the Chicago Cubs. With the Dodgers leading, 5-3, and two outs in the top of the ninth inning, the Cubs' Jason Heyward was on second base. Concerned that a runner at second base could possibly steal signs, Jansen intentionally balked, advancing the runner to third base. Jansen then struck out batter Víctor Caratini for the final out of the game.