Probability begins with the notion of a random experiment. This chapter describes the notion of probability. It shows how probability can be computed as a relative frequency using simulation. Probabilities must be assigned to outcomes in such a way that they add up to one. Once people have a valid assignment of probabilities to outcomes, the probability of an event is simply the sum of the probabilities of the individual outcomes making up the event. The chapter shows how to compute probabilities programmatically. A large number of samples will be needed before the mean stabilizes and hits the true mean. The true mean is referred to also as the population mean. The function round is not explicitly included into the program. This is because it is a built-in function. The chapter also shows how to build simulation models for simple random experiments.