In baseball or softball, a starting pitcher (also referred to as the starter) is the pitcher who delivers the first pitch to the first batter of a game. A pitcher who enters the game after the first pitch of the game is a relief pitcher.
A starting pitcher in professional baseball usually rests three or four days after pitching a game, before pitching another. Therefore, most professional baseball teams have four or five starting pitchers on their rosters. These pitchers, and the sequence in which they pitch, is known as the rotation. In modern baseball, a five-man rotation is most common.
Under ideal circumstances, a manager of a baseball team would prefer a starting pitcher to pitch as many innings as possible in a game. Most regular starting pitchers pitch for at least five innings on a regular basis, and if a pitcher is unable to do so, there is a high probability that he will, in the future, be relegated to duty in the bullpen. In modern baseball, a starting pitcher is rarely expected to pitch for more than seven or eight innings, at which point, responsibility for the game is passed to relief pitchers, including specialist pitchers such as set-up pitchers and closers.
Often, a starting pitcher is subject to a pitch count, meaning the manager will remove him from the game once he has thrown a specific number of pitches. The most common pitch count for a modern pitcher is in the neighborhood of 100, and it is now rare for a starting pitcher to throw more than 125 pitches in a game. Pitch count limits are especially common for starting pitchers who are recovering from injury. In the early decades of baseball, it was not uncommon for a starting pitcher to pitch three hundred innings or more, over the course of a season. In addition, there are accounts of starting pitchers pitching on consecutive days, or even in both games of a doubleheader. It is believed that these feats were only possible because pitchers in the early years of the game, unlike modern starters, rarely threw the ball with maximum effort.
A starting pitcher who can be counted on to consistently throw many innings is known as a workhorse. A good example of a modern day workhorse pitcher is Roy Halladay, who is the current active leader in both complete games thrown and shutouts.
A starting pitcher must complete five innings of work in order to qualify for a "win" in a game he starts. Under NCAA baseball rules, which govern intercollegiate baseball, a starting pitcher who pitches fewer than five innings can still earn a win if he pitches for a certain amount of time that is determined before the start of the game. It is possible to be credited with a loss despite pitching fewer than five innings. A starter who works six or more innings while giving up three or fewer earned runs is said to have achieved a "quality start". A starter that finishes the game without having to be relieved by the bullpen is said to have thrown a "complete game". The pitcher that throws a complete game is almost always in a position for a win.
Pitch selection 
Starting pitchers usually have a variety of pitches to choose from, broken into a number of categories.
Fastballs: A pitch thrown hard (anywhere from the upper 80s to over 100 mph) and which generally follows a mostly straight trajectory. There are a number of different types of fastballs. The 4-seam fastball is the hardest thrown pitch, but also has very little movement or break to it. The 2-seam fastball is slightly slower than the 4-seam (generally in the mid 80s to low 90s), but breaks slightly inward to the pitcher's throwing arm (i.e., a left-handed pitcher throwing a 2-seam fastball will have it tail slightly left-to-right). The cut fastball (cutter) is similar to the 2-seam in the direction of its break and velocity, but like a slider it breaks much later in its flight path, though not as sharply and travels faster than that breaking ball.
Hard breaking balls: The most prominent of the hard breaking balls is the slider. A slider is a pitch that breaks sharply in the direction of the pitchers arm travel (left to right for a left-handed pitcher). It travels slower than a fastball (usually in the upper 70s to mid 80s) but faster than the slower breaking balls. The other two hard breaking balls are variants of the fastball, the sinker and the split-finger fastball (splitter). Both of these pitches break downwards from the point of release, with the sinker generally tailing slightly to the pitchers arm travel and the splitter tailing opposite. Both pitches are usually thrown in the low to upper 80's, although some travel upwards of 90 mph. The major difference is in their grip and the timing of their break. Like the cut fastball to the 2-seamer, the splitter tends to break much later in its flight path than the sinker does.
Soft breaking balls: The most common soft breaking ball is the eponymous curveball. The curve breaks in the direction of the pitchers arm travel from the point of release on through the entire arc of its flight. Curveballs travel from the low 60s low 80s in speed. The other soft curveball is known as the screwball. The screwball is a fairly rare pitch in modern baseball, as its manner of throwing puts a lot of stress on the arm and today's maximum velocity/maximum effort pitchers can tear up their arms trying to use this pitch. The screwball is essentially a reverse curve ball, as it breaks in the opposite direction of the pitchers arm travel.
Other pitches: Two other major pitch styles are used by pitchers today, one far moreso than others. The changeup, or palmball, is a slow pitch that is thrown with the same arm motion and arm velocity of a fastball, but with a much different grip that keeps the ball from achieving the same speed, usually the changeup is 10-20 mph slower than the pitcher's fastballs. This visual distortion from a fast arm swing and a slower pitch is used to disrupt the hitter's timing. The other major pitch style is the knuckleball. The knuckleball is a very difficult pitch to master, for both the pitcher (due to its unique grip and delivery manner) and for his catcher (due to the pitch being thoroughly unpredictable in its travel).
See also 
- Setup man
- Middle reliever
- Closing pitcher
- Left-handed specialist
- Long reliever
- List of World Series starting pitchers
- For an evaluation of the relative merits of a four-man and a five-man rotation, see Rany Jazayerli, "Doctoring The Numbers: The Five-Man Rotation, Part 3," BaseballProspectus.com (August 30, 2002).