Choosing Bowlers.

At the beginning of each over, the captain must decide which of the players on his side will bowl the over. Bowlers generally perform best in spells of 5-10 overs, which gives them time to bowl several balls and settle into a good rhythm, and then allows them to rest after bowling for 20-40 minutes. Because of the rule which prohibits a bowler from bowling two successive overs, a single bowler will bowl every second over from one end of the pitch, while a team-mate bowls the intervening overs from the other end.

The particular bowler chosen to bowl an over depends on several factors:

The age of the ball.
The relative skills of the available bowlers in the side.
The bowling styles of the bowlers.
The freshness or tiredness of all the potential bowlers.
Whether a bowler has been performing well or poorly so far in the game.
The state of the pitch, and whether it has deteriorated over the course of the game.
Any known strengths or weaknesses of the batsmen to particular bowlers or styles of bowling.
The state of the game and overall strategic position, in terms of whether it is more important to take wickets or prevent runs.
The wind speed and direction.
The temperature and humidity.
The time of day, in terms of whether it is near the end of a session of play.

The age of the ball often dictates which style of bowler the captain will choose. Fast bowlers obtain greater bounce and movement through swing and seam with a new ball than an older one. Almost invariably, fast bowlers will bowl until the ball as at least 20-25 overs old. The captain usually opens the attack with his best two fast bowlers bowling initial spells in alternation, then substitutes other bowlers as the opening pair of bowlers become tired.

Once the ball has begun to wear significantly, spin bowlers can get significant spin off the pitch and will come into play. If the ball is getting very old (50 overs or more) and begins to reverse swing, any fast bowler who can produce reverse swing will become a good option.

If the pitch is green, containing significant moisture, as is sometimes the case early on the first day of a match, fast bowlers can get variable bounce which makes the ball difficult for the batsman to judge and play. High humidity increases the swing of the ball, and this is affected by moisture evaporating from the pitch, so the combination of a moist pitch and humid weather can make fast bowlers very effective.

On the other hand, spin bowlers gain most assistance from the pitch when it is dry and begins to crumble, break up, and become dusty in character. Cracks in the surface and the lifting of patches of dead grass make the pitch uneven and produce variable amounts of spin and bounce, which makes spin bowling difficult to play. On the last day of a match it is often spin bowlers who are called on to perform most of the bowling.

Wind speed and direction can be a factor for choosing which end of the pitch best suits a bowler. Fast bowlers who rely more on sheer speed than swing gain assistance by bowling with the wind. Swing bowlers are helped by a crosswind that blows in the same direction as they are trying to swing the ball. Spin bowlers generally prefer bowling into the wind, as it helps slow the ball down and give it more time for drift and flight before bouncing. A crossbreeze component in the right direction can also help a spin bowler's natural drift.

Finally, if it is the last over or two before a break in play, the captain may choose to use a part-time bowler, more for the variety and the possibility that a batsman will attempt a rash shot and accidentally get out than for his actual bowling skill.