Epidemiological studies are either observational or experimental. The choice of study design depends on the populations available for study, the frequencies of exposures and outcomes in the sample population, and the hypothesis or study question being addressed. Each study design has strengths and weaknesses and should be carefully matched to the type of data collected and the desired information. Various study designs exist, including experimental, quasi-experimental, descriptive, analytic, and others that are discussed in this chapter. Systematic reviews and meta-analyses are used to summarize evidence related to a specific research question. The critical evaluation of published research, including the type of study design used, is necessary to appropriately assign value to the conclusions of the authors of a paper.