Water polo, or Water ball, is a team water sport. The playing team consists of six field players and one goalkeeper. The winner of the game is the team that scores the most goals. Game play involves swimming, treading water (using a sort of kicking motion known as "eggbeater kick"), players passing the ball while being defended by opponents, and scoring by throwing the ball into a net defended by a goalie. 'Man-up' (or 'power play') situations occur frequently. Water polo, therefore, has strong similarities to the land-based game of handball.

Seven players from each team (six field players and a goalkeeper) are allowed in the playing area of the pool during game play with up to four substitutes. Visiting team field players wear numbered and usually White caps, and home team field players wear usually Blue caps (though any other contrasting colors are now allowed); both goalies wear red caps, numbered "1". Players may be substituted in and out after goals, during timeouts, at the beginning of each quarter, after ordinary fouls and after injuries. During game play, players enter and exit in the corner of the pool, or in front of their goal; when play is stopped, they may enter or exit anywhere.