Scientific rigour in the design of sample surveys is crucial to ensure that the final results are accurate and representative. If the sampling design is of poor quality, the value of the survey will be undermined. The chapter is structured in four major sections: • Sampling methodology – basic concepts. This section explains why cluster sampling is the optimal sampling design for prevalence surveys • Calculation of sample size. This section describes the key components of a sample size calculation, and shows step-by-step how to calculate the sample size that is required. Important concepts such as relative precision and the design effect are defined and discussed • Selection of clusters and selection of individuals within clusters. This section covers the definition of a cluster, the role of stratification, and the practical steps needed to select first clusters and then individuals from within a cluster • Definition of the eligible survey population.This section explains how to define the eligible survey population, and why this is critical to the estimation of the true country-wide prevalence of TB.