A bookmaker, or bookie, is an organization or a person that takes bets and pays winnings depending upon results and, depending on the nature of the bet, the odds.

Most bookmakers in the United States bet merely on college and professional sports, though in the United Kingdom they offer a wider range of bets, including each-way betting on golf, football and tennis, and especially horse racing and greyhound events. They also specialize in novelty events such as betting the probability that it will snow on Christmas Day, the outcome of political elections or reality television contests.

Operational procedures
By adjusting the odds in his favor or by having a point spread, the bookmaker will aim to guarantee a profit by achieving a 'balanced book', either by getting an equal number of bets for each outcome, or (when he is offering odds) by getting the amounts wagered on each outcome to reflect the odds. When a large bet comes in, a bookmaker can also try to lay off the risk by buying bets from other bookmakers. The bookmaker does not generally attempt to make money from the bets themselves, but rather profiting from the event regardless of the outcome. Their working methods are similar to that of an actuary, who does a similar balancing of financial outcomes of events for the assurance and insurance industries.

Bookmaking may be legal or illegal, and may be regulated; in the United Kingdom it was at times both regulated and illegal, in that licences were required but no debts arising from gambling could be enforced through the courts. Now, since the inception of the National Lottery, not only is it completely legal in the UK, it is a small contributor to the British economy, with a recent explosion of interest with regard to the international gaming sector industry. However, gambling debts still remain unenforceable under English law.

Bookmaking is generally illegal in the United States, with Nevada being a notable exception. In some countries, such as Singapore, The Netherlands Sweden and Canada, the only legal bookmaker is state-owned and operated. In the United Kingdom, trusted legal bookmakers are members of IBAS, which is an industry standard organization which resolves to settle disputes.

United Kingdom gambling industry
Traditionally, bookmakers have been located at the racecourse, but improved TV coverage and modernisation of the law have allowed betting in shops and casinos in most countries. In 1961, Harold Macmillan's Conservative Government legalised betting shops and tough measures were enacted to ensure that bookmakers remained honest. A large and respectable industry has grown since. At one time there were over 15,000 betting shops in the UK. Now, through consolidation, they have been reduced to about 8,500. Currently there are four major "high street" bookmakers in the United Kingdom: William Hill, Ladbrokes, Coral, and state-owned ToteSport, Bet24, Betfred, Victor Chandler, Stan James, Sportingbet, and Bet365, rapidly emerging, in terms of turnover and event sponsorship.

Internet gambling
With the arrival of the internet, many bookmakers have an online brand, although independently owned bookmakers often still maintain a "bricks and mortar" only operation and others operate a "skin" or "white label" operation which they purchase from one of the large firms as is the case with BetDirect and Betterbet. The main websites only accept bets from countries where internet gambling is not prohibited, and from people over 18 years old. Often these websites are linked to online casinos. Controversially, the explosion in Internet gambling is being linked to an increase in gambling addiction, according to the UK's help and advice organizations for addicts, GamCare and Gamblers Anonymous.

Increasingly, online bettors are turning to the use of betting exchanges such as Betsson and Betfair, which automatically match Back and Lay bets between different bettors, thus effectively cutting out the bookmaker's traditional profit margin also called an overround.

These online exchange markets operate a market index of prices near but usually not at 100% competitiveness as exchanges take commissions on winnings. True Wholesale odds are odds that operate at 100% of probabilistic outcomes.

Some bookmakers have even taken to using betting exchanges as a way of laying off unfavorable bets and thus reducing their overall exposure. This has led insecurity from the TAB in Australia, a government-run betting agency which attempted to deny Betfair an Australian license by running unfavorable ads in the media regarding the company. When Tasmania granted Betfair a license despite these efforts the Western Australian state legislature passed a law that specifically criminalised using betting exchanges from within the state, however that law was later ruled to be unconstitutional.

Betting exchanges are universally disliked by the traditional bookmaker. Not only are they generally able to offer punters better odds due to their much lower overheads, but also in giving opportunities for arbitrage: the practice of taking advantage of a price differential between two or more markets, although traditionally arbitrage has always been possible by backing all outcomes with bookmakers (dutching) as opposed to laying an outcome on an exchange. Exchanges do, however, allow bookmakers to see the state of the market and can set their odds accordingly.

Most televised sport in the United Kingdom and Europe is now sponsored wholly or partly by Internet and high street bookmakers, with sometimes several bookmakers and online casinos being displayed on players' shirts, advertising hoardings, stadium signs and competition event titles, although Werder Bremen are currently fighting the German courts for the freedom to continue featuring bookmaker Bwin on their shirts, as Germany and France take action against online gamers.

The United Kingdom Gambling Act 2005 introduces a new regulatory system for governing gambling in Great Britain. This system includes new provisions for regulating the advertising of gambling products. These provisions of the Act came into effect in September 2007. It is an offence to advertise in the UK, gambling which physically takes place in a non-European Economic Area (EEA), or in the case of gambling by remote means, gambling which is not regulated by the gambling laws of an EEA state.

The situation is more confused in the United States, which has attempted to restrict operators of foreign gambling websites accessing their domestic market. This has resulted in a ruling against the US Government by the WTO.