A base on balls (BB), also known as a walk, occurs in baseball when a batter receives four pitches that the umpire calls balls, and is in turn awarded first base without the possibility of being called out. The base on balls is defined in Section 2.00 of baseball's Official Rules, and further detail is given in 6.08(a). It is considered a faux pas for a professional player to literally walk to first base; the batter-runner and any advancing runners normally jog on such a play.

The term "base on balls" distinguishes a walk from the other manners in which a batter can be awarded first base without liability to be put out (e.g., hit by pitch (HBP), catcher's interference). Though a base on balls, catcher's interference, or a batter hit by a pitched ball all result in the batter (and possibly runners on base) being awarded a base, the term "walk" usually refers only to a base on balls, and not the other methods of reaching base without the bat touching the ball. An important difference is that for a hit batter or catcher's interference, the ball is dead and no one may advance unless forced; the ball is live after a walk (see below for details).

A batter who draws a base on balls is commonly said to have been "walked" by the pitcher. When the batter is walked, runners advance one base without liability to be put out only if forced to vacate their base to allow the batter to take first base. If a batter draws a walk with the bases loaded, all preceding runners are forced to advance, including the runner on third base who is forced to home plate to score a run; when a run is forced on a walk, the batter is credited with an RBI per rule 9.04.

Receiving a base on balls does not count as a hit or an at bat for a batter but does count as a time on base and a plate appearance. Therefore, a base on balls does not affect a player's batting average, but it can increase his on-base percentage.

A hit by pitch is not counted statistically as a walk, though the effect is mostly the same, with the batter receiving a free pass to first base. One exception is that on a HBP (hit-by-pitch), the ball is dead. On a HBP, any runners attempting to steal on the play must return to their original base unless forced to the next base anyway. When a walk occurs, the ball is still live: any runner not forced to advance may nevertheless attempt to advance at his own risk, which might occur on a steal play, passed ball, or wild pitch. Also, because a ball is live when a base on balls occurs, runners on base forced to advance one base may attempt to advance beyond one base, at their own risk. The batter-runner himself may attempt to advance beyond first base, at his own risk. Rule 6.08 addresses this matter as well. An attempt to advance an additional base beyond the base awarded might occur when ball four is a passed ball or a wild pitch.