Umpires have a number of responsibilities that take place before the first pitch is even thrown. It is the umpire's responsibility to make sure the base lines and batter's boxes are clearly marked in white chalk and to ensure the home team supplies enough balls to last throughout the game.
The home plate umpire's primary responsibility is to call balls and strikes. While baseball is not generally a fast-paced sport, calling balls and strikes is never easy since the umpire must make split-second decisions. Although umpires differ in their interpretation of the strike zone, it is important for individual umpires to remain consistent from pitch to pitch and game to game. With pitches coming to the plate at speeds approaching 100 mph and curving in every direction, it can be extremely difficult to make the right call immediately after the pitch crosses the plate.
Umpires must also determine whether runners are safe or out at each base and whether batted balls are fair or foul. At the professional level, there are umpires at each base to make this decision, but at lower levels there is commonly one field umpire who is responsible for calling outs at each base.
Having accepted the lineup cards from both team prior to the start of the game, the umpire is required to keep track of any lineup changes and substitutions that occur over the course of the game.
Umpires at all levels are required to use their best judgment to maintain order between teams. To help do so, umpires can issue warnings to teams (for example, if a pitcher is deemed to have intentionally thrown at a batter) and, if the situation warrants, eject players, coaches or managers from the game.
With a growing number of controversial calls in recent years, umpires at the major league level have been given the advantage of limited instant replay to review questionable calls. This modern use of technology often has resulted in reversed calls.