This chapter explores sampling principles and techniques essential for conducting epidemiological research. It begins with an overview of populations in research, distinguishing between random sampling and random allocation. Various sampling methods are then described, including convenience sampling, systematic random sampling, simple random sampling, stratified random sampling, and cluster random sampling. The chapter highlights the differences between stratified and cluster sampling and introduces stratified cluster sampling as a combined approach. For each method, key features, applications, advantages, and limitations are discussed, providing readers with the knowledge to choose the most appropriate sampling strategy for different epidemiological study designs.