While some casino games have a skill element in the outcome, some casino games are purely based upon chance. Because all best slots online 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 exactly what it says: generates a completely random number. Not all RNGs are identical, and they have to be tested extensively to make sure that the numbers they generate really are random.

In some games, the utilization of an RNG has an obvious role. In European roulette, as an 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.