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

In certain games, the use of an RNG has an obvious role. In European roulette, for instance, 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.