Physical contact is the rule rather than the exception in water polo. More than most team sports, water polo is played under the constant control of the referees. Whistles blow incessantly during a match, and the foul rules are the most important aspect of the modern game.

Fouls account for about 90 percent of the whistles. Infractions include touching the ball with two hands (only the goalie is allowed to do this); dunking the ball underwater; gripping an opponent by the swimsuit; punching the ball; pushing off an opponent; and deliberately impeding an opponent's movement when he doesn't have the ball.

Most common fouls are called on defensive players, but the rules apply equally to players on offense. The fouled team gains possession and has to put the ball in play without unreasonable delay and without taking a direct shot on goal. In general, the common foul allows the team on offense an opportunity to complete the original pass.

There is a three-second grace period after a common foul and the resulting free throw. If a defensive player fouls during those three seconds, he is charged with a 20-second penalty. The attacking team then has a man-advantage power play.

A foul is called after a serious infraction. Fouls include kicking or hitting an opponent; deliberately splashing water in an opponent's face; a common foul committed during dead time; interfering with a free throw; misconduct or showing disrespect to the referee; and holding, sinking or pulling an opponent not holding the ball. A major foul results in either a 20-second penalty on the guilty player or a penalty shot.

A major foul by a defender not drawing a penalty shot requires the offending player to leave the pool immediately. His team plays short-handed until the penalty expires -- unless either the opposing team scores a goal or the penalized player's team regains possession of the ball. This is when things get exciting!

After being called for three major fouls, a player is disqualified and must be replaced in the pool. Brutality fouls, involving a clear attempt to injure, result in the expulsion of the offending player for the remainder of the match, with no substitute allowed. A goalie drawing a major foul has to serve their own penalty.

A penalty shot is awarded when either an act of brutality or a foul that prevents a probable goal occurs within the four-meter zone. Anyone but the goalkeeper can take the penalty shot, which is thrown from the four-meter line (13 feet, 1 inch). About 85 percent of penalty shots score. There are two referees -- each patrolling a side of the pool on elevated platforms -- and two goal judges, one at each goal line.

On the international level, the referee's judgment is of enormous importance. Referees indicate fouls by blowing their whistles. The color of the flag held up indicates which team has the ball in change-of-possession situations.