Fixed-odds betting is a form of wagering against odds offered by a bookmaker or an individual or on a bet exchange. It involves betting on an event in which there is no fluctuation on the payout. In Australia, the practice is usually known as "SP betting".

Fixed-odds gambling involves placing bets on events with predetermined odds. Bookmakers aim to create an overround, where the sum of probabilities quoted for all possible outcomes exceeds 100%, ensuring profit. Imbalanced books can occur, leading to higher or lower payouts than expected. The advent of the internet and betting exchanges has led to opportunities for fixed-odds arbitrage actions and Dutch books.

When a bet has a positive expected value, it is said to be getting "the best of it." In contrast, "laying odds" refers to a bet in which more is risked than can be won, and rational bettors only engage in this type of bet if the chances of an adverse outcome are low enough. "Lay betting" is when a bettor bets against a specific outcome, effectively taking on the role of a bookmaker.

Odds can be expressed in various formats, including fractional, decimal, and moneyline. Fractional odds are used primarily in the United Kingdom and Ireland, while decimal odds are favored in Continental Europe, Australia, New Zealand, and Canada. Moneyline odds are used in the United States. Converting between these formats requires specific calculations depending on the type of odds used.