In a normal over, a single bowler delivers six balls in succession, from one end of the pitch, to the batsman at the other end. After six deliveries, the umpire calls 'over'; the fielding team switches ends, and a different bowler is selected to bowl an over from the opposite end of the pitch, while the batsmen do not change ends. The captain of the fielding team decides which bowler will bowl any given over, and no bowler may bowl two overs in succession.
An over must consist of six legal deliveries. If the bowler bowls a wide or a no ball, that illegal delivery is not counted towards the six-ball tally, and another delivery will need to be bowled in its place.
In the event that a bowler is injured, or is sent out of the attack by the umpire (for disciplinary reasons, such as bowling beamers), during the middle of an over, a team-mate completes any remaining deliveries.
Because a bowler may not bowl consecutive overs, the general tactic is for the captain to appoint two bowlers to alternate overs from opposite ends. When one bowler tires or becomes ineffective, the captain will replace that bowler with another. The period of time during which a bowler bowls every alternate over is known as a spell.
In limited overs cricket matches, such as one-day cricket and Twenty20, each team has one batting innings, which ends either after all wickets are lost or when the allotted overs are completed. In such a match, bowlers are generally restricted to the total number of overs they may bowl in a match. The general rule is that no bowler can bowl more than 20% of the allotted overs per innings; thus, in a 50 overs match, each bowler can bowl up to a maximum of 10 overs.
In Test and first class cricket, there is no limit to the number of overs which may be completed in a team's innings, nor is there any limit to the number of overs which may be bowled by a single bowler. In these matches, there is a requirement to bowl a minimum of ninety overs in a day's play, to ensure a good spectacle, and to prevent the fielding team from wasting time for tactical reasons.
A maiden over is one in which no runs are scored. Leg byes and byes scored in the over are not counted against the bowler in a maiden over. A wicket maiden is one in which no runs are scored and a wicket is taken: double and triple wicket maidens have also been recorded.
Tactical considerations in bowling overs 
The over is a fundamental consideration in the tactical planning of the fielding side. Since a single bowler has only six legal balls to bowl before he must hand the ball to another bowler, he typically plans to use those six balls to set up a pattern of play designed to get a batsman out. For example, he may bowl the first few balls with the same line, length, or spin. He intends to tempt the batsman into scoring runs by providing balls that are relatively easy to hit. If the batsman takes the bait, the bowler can then follow up with a variation designed to hit the wicket, or a ball that is intended to induce a mistake from a batsman who is still in aggressive run-scoring mode, which will result in him being caught out. The latter type of delivery is known as a sucker ball.
Cricket imposes penalties on captains if their players bowl their overs at a very slow rate, such as fines, loss of competition points, and match bans. If a team is proceeding slowly, some captains will choose to use slow/spin bowlers. Such bowlers have a shorter run up so they complete their overs more quickly. Often this means choosing an inferior strategy by employing a less skillful bowler to avoid penalties that are perceived to be greater, such as being banned or losing points.
Tactical considerations in batting 
If the two batsmen are not similar, tactical considerations may affect their play. If one batsman is stronger than the other, they may attempt to engineer their scoring so that the stronger batsman faces the bowling more often. This may take the form of the stronger batsman trying to score an even number of runs on early balls in the over and an odd number on the last ball; the weaker batsman will attempt the reverse, and the bowler will try to disrupt this pattern.
If one batsman is right-handed and the other left-handed, they may try to score odd numbers of runs to disrupt the bowling pattern and tire the fielders by making them reposition themselves frequently.
Historical number of balls per over in Test cricket 
Since 1979/80, all Test cricket has been played with six balls per over. However, overs in Test cricket originally had four balls per over, and there has had varying number of balls per over around the world up to 1979/80, generally the same as the number of balls per over in force in other first-class cricket in that country.
Balls per over
- 1880 to 1888: 4
- 1889 to 1899: 5
- 1900 to 1938: 6
- 1939 to 1945: 8 (though not in the "Victory" Tests)
- 1946 to date: 6
- 1876/77 to 1887/88: 4
- 1891/92 to 1920/21: 6
- 1924/25: 8
- 1928/29 to 1932/33: 6
- 1936/37 to 1978/79: 8
- 1979/80 to date: 6
In South Africa
- 1888/89: 4
- 1891/92 to 1898/99: 5
- 1902/03 to 1935/36: 6
- 1938/39 to 1957/58: 8
- 1961/62 to date: 6
In New Zealand
- 1929/30 to 1967/68: 6
- 1968/69 to 1978/79: 8
- 1979/80 to date: 6
- 1954/55 to 1972/73: 6
- 1974/75 to 1977/78: 8
- 1978/79 to date: 6
- The Laws of Cricket (2000 Code 4th Edition - 2010) Marylebone Cricket Club, Law 22 THE OVER
- The Laws of Cricket, Law 42.9 "Time wasting by the fielding side"
- Cricket: A History of its Growth and Development throughout the World. Rowland Bowen. Eyre & Spottiswoode (1970). v. Index entry "Overs", p409
- Bowen, p348