Fixed-odds gambling is a form of gambling against odds offered by a
bookmaker, an individual, or on a
betting exchange. It is customary with fixed-odds gambling to know the expected odds at the time of the placement of the wager, however this category also accommodates events in which wagers are struck at "starting price". It is typical for a bookmaker to price up a book such that the net outcome will always be in his favour, i.e. the sum of all possible outcomes will be in excess of 100%. The amount of the sums wagered in excess of 100% represent profit to the bookmaker in the event of a balanced book. This excess is known commonly as an overround.
With the advent of internet and exchange betting, the possibility of fixed-odds arbitrage actions against bookmakers and exchanges has opened up significantly.