A random-number generator (RNG) is a computational or physical device designed to generate a sequence of numbers or symbols that can not be reasonably predicted better than by a random chance.

Various applications of randomness have led to the development of several different methods for generating random data, of which some have existed since ancient times, among their ranks are well known "classic" examples, including the rolling of dice, coin flipping, the shuffling of playing cards, the use of yarrow stalks (for divination) in the I Ching, as well as countless other techniques. Because of the mechanical nature of these techniques, generating large numbers of sufficiently random numbers (important in statistics) required a lot of work and/or time. Thus, results would sometimes be collected and distributed as random number tables. Nowadays, after the advent of computational random-number generators, a growing number of government-run lotteries and lottery games have started using RNGs instead of more traditional drawing methods. RNGs are also used to determine the odds of modern slot machines.