This paper, authored by Bruce D. Meyer, explores the use of natural and quasi-experiments in economic research, which involves examining outcome measures for treatment and comparison groups that are not randomly assigned. These studies leverage variations in explanatory variables generated by changes in state laws, government policies, or other factors to obtain exogenous variation. The paper highlights the advantages of these studies and suggests ways to improve their design, including the use of multiple treatment and comparison groups and multiple pre- or post-intervention observations. It also discusses methods for assessing the validity of inferences drawn from such studies, such as examining comparability of groups and probing the implications of hypotheses. The paper provides examples from various economic fields, including social insurance programs, military service, minimum wage laws, and immigration, to illustrate the application of natural experiments. Additionally, it addresses common threats to validity, such as omitted variables, selection bias, and attrition, and offers strategies to mitigate these issues. The paper concludes by emphasizing the importance of understanding the sources of exogenous variation and the need for careful design to ensure the reliability of economic conclusions.This paper, authored by Bruce D. Meyer, explores the use of natural and quasi-experiments in economic research, which involves examining outcome measures for treatment and comparison groups that are not randomly assigned. These studies leverage variations in explanatory variables generated by changes in state laws, government policies, or other factors to obtain exogenous variation. The paper highlights the advantages of these studies and suggests ways to improve their design, including the use of multiple treatment and comparison groups and multiple pre- or post-intervention observations. It also discusses methods for assessing the validity of inferences drawn from such studies, such as examining comparability of groups and probing the implications of hypotheses. The paper provides examples from various economic fields, including social insurance programs, military service, minimum wage laws, and immigration, to illustrate the application of natural experiments. Additionally, it addresses common threats to validity, such as omitted variables, selection bias, and attrition, and offers strategies to mitigate these issues. The paper concludes by emphasizing the importance of understanding the sources of exogenous variation and the need for careful design to ensure the reliability of economic conclusions.