Betting exchanges are again hitting the headlines following the arrest of champion jockey Kieren Fallon as part of an investigation into the fixing of horse races.
It is reported that the betting exchange Betfair contacted the Jockey Club last year with concerns over a number of races, and the evidence was handed to City of London Police who began a formal investigation.
On Wednesday, police arrested 16 people, including Fallon, two other jockeys and a racehorse trainer.
BBC Sport looks at the issues surrounding gambling's newest phenomenon and examines the implications for the world of racing.
WHAT ARE BETTING EXCHANGES?
They are online betting sites that have rapidly developed over the past three years where punters can bet against each other.
Punters can either back a horse to win or "lay" a bet that a horse will lose at their chosen odds.
The exchange acts as a broker, matching the punter with the layer, and takes a commission for this service.
WHAT ARE THE ADVANTAGES?
Punters get the choice of betting in the normal way or playing bookmaker, controlling their odds.
Exchanges also provide the opportunity to allow punters to bet "in-running" - after the race has started.
SO WHY ARE THEY CRITICISED?
Racing bosses are concerned that betting exchanges represent a threat to the integrity of the sport.
Former British Horseracing Board chairman Peter Savill said: "Betting exchanges have suddenly enfranchised 30m-plus people in Britain to make money out of horses losing a race.
"When you add to that figure every other person in the world with the desire to make money out of horses in Britain losing races - including, possibly, illegal Far East bookmakers and even organised crime - you have to wonder whether the decision [to allow franchises] was reached after appropriate research."
WHAT DO THE EXCHANGES SAY?
They insist that they have increased the transparency in betting activities through the signing of the Memorandum of Understanding with the Jockey Club in June, 2003.
Under the agreement, exchanges pass on information relating to unusual or suspicious betting patterns to the Jockey Club.
Rob Hartnett, a spokesman for betting exchange Betdaq, highlighted the ability to provide a paper trail of the activities of members.
"With an online exchange, there is a record of every bet struck, and all the betting exchanges have worked with the Jockey Club and other sporting authorities to make sure that the sport is run properly and cleanly," he said.
MMIX
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Racing in crisis