Abstract: Online betting systems are disclosed in which a number of players may offer and accept bets online on the outcome of events, for example sporting events. Each player may be both offering bets and accepting them. By linking together, in a central server operating the system, databases reflecting betting data relating to bets which have been both offered and accepted, but which are outstanding because the outcome of the event is not yet known, and reflecting account data of the players, a players theoretical exposure at any one time may be calculated, and action taken to update the betting database data if the theoretical exposure would exceed the amount held in the account of the player concerned. By carrying out that calculation ignoring unmatched offers which each player has made, unnecessary restrictions do not need to be placed on the offers made by each player. Once an offer is accepted, however, a fresh calculation is made and this can result in previously existing other offers being deleted.