The sample size is an important feature of any empirical study in which the aim is to make inferences about a population from a sample. One of the most frequent problems in planning a survey and a critical component in study design is the determination of the appropriate sample size; indeed, it is more crucial than statistical analysis. The determination of the appropriate sample size involves statistical criteria as well as clinical or practical considerations. In this chapter, different sample size estimation formulas will be presented and illustrated with examples for a better understanding. In addition, sampling and nonsampling errors will be discussed in detail.