While some casino games have a skill element in the outcome, some casino games are purely according to chance. Because all good online casino casino games are partly or completely driven by chance, all casino games depend on a random number generator, or RNG within the software that powers the game. The RNG is a mathematical application that does precisely what it says: generates a totally random number. Not all RNGs are the same, and they have to be tested extensively to guarantee that the numbers they generate really are random.

In some games, the usage of an RNG has an obvious role. In European roulette, for example, the RNG generates a number from 1 to 37 (with 37 corresponding to "0") on the roulette wheel. The RNG generates a number, the casino stops, and bets are paid or forfeited.

In craps, the RNG must generate two random numbers simultaneously, and in single deck card games, the RNG must generate a number from 1 to 52, with each of those numbers corresponding to a particular card.