A system for detecting collusion in a game having a plurality of participating players comprises means for recording, for each player, an amount wagered on each turn of the game in which the player participates, and a corresponding outcome of said wager, the outcome being a complete or partial forfeit of the wager if the wager is unsuccessful, and a profit made on the wager if the wager is successful; a ranking facility operable to derive a tertiary statistic for each pair of participating players, the tertiary statistic being a function of the cumulative wagers by each player in the pair in all turns of the game in which both players have wagered, and the cumulative outcomes of the wagers made by each player in the same turns of the game; and monitoring means for monitoring the tertiary statistic of each pair of players and generating an output when the tertiary statistic exceeds a predetermined threshold, the output being an indicator of possible collusion by the respective pair of players.