The Elo rating system is a method for calculating the relative skill levels of players in two-player games such as chess. It is named after its creator Arpad Elo, a Hungarian-born American physics professor.

The Elo system was invented as an improved chess rating system, but today it is also used in many other games. It is also used as a rating system for multiplayer competition in a number of video games, and has been adapted to team sports including association football, American college football, basketball, and Major League Baseball.

Each player has a rating, which is a number. A higher number indicates a better player, based on their results against other rated players. The winner of a contest between two players gains a certain number of points in his rating and the losing player loses the same amount. The number of points won or lost in a contest depends on the difference in the ratings of the players, so a player will gain more points by beating a higher-rated player than by beating a lower-rated player. In chess, for instance, if one player is rated 100 points higher than the other player, he is expected to win about five games out of eight, and the rating changes reflect that. Over a series of games, if a player does better than expected, based on the ratings (his compared to his opponents), his rating will go up.