This contribution is devoted to the theoretical foundations of sampling. It is organised as follows: First, the reader is introduced to sampling terminology; terms such as (theoretical) population, accessible population, and sampling frame are discussed. The different types of random samples are then addressed individually. When doing so, the formulae are presented for simple random, stratified, systematic, and cluster sampling; for two-stage sampling procedures; and for sampling procedures with unequal inclusion probabilities. The contribution concludes by explaining how the necessary sample size is determined.