"In May of 1963...the FBI turned over to the Justice Department a two-volume document called "The Skimming Report," which detailed the illegal siphoning off of gambling profits by Las Vegas casinos to avoid taxes."[1] The report documented how pre-tax profits from casinos were being routed to various organized crime syndicates across the nation. Unfortunately no one could be prosecuted as the FBI obtained its information by illegally bugging casino money rooms. The money rooms are where the cash from the betting floor is counted and recorded on the casino's books.
Skimming by diverting pre-tax profits is just one of many possible methods. In casinos the patron is betting against the house, so the house has an incentive to fix the games (cards, roulette, slots, etc.). In pari-mutuels (horse and dog track, jai alai) the house instead gets a fixed percentage of the total amount bet, so is theorectically less likely to fix the games. However, pari-mutuels use sophisticated computer systems to handle customer bets and a major scandal involving Autotote, a major supplier of these systems, has been exposed.[2]
Other methods of skimming, such as arranging for particular employees of organized crime be allowed to win in rigged games, was sometimes used.