In this chapter, we cover the fundamentals of sampling theory, the how and why of sample selection. In the first section we discuss the aims of sampling. We then move on to definitions and a discussion of fundamental concepts of sampling theory-population, the sampling unit, sampling frame, and the sample-as well as the construction of probability and nonprobability sampling designs. Next, we discuss the factors relevant to our choice of the appropriate sample size. Finally, we present procedures for estimating nonsampling errors