This chapter examines the principles and methods used to evaluate the performance of diagnostic and screening tests in epidemiology. It begins with the concept of validity, emphasizing the importance of accurately identifying individuals with and without a disease. Key measures such as sensitivity and specificity are defined and their applications discussed. The chapter then explores strategies involving multiple tests, including sequential and simultaneous testing, highlighting their advantages and limitations. Measures of test performance in practical settings—positive predictive value (PPV) and negative predictive value (NPV)—are presented, along with an analysis of how prevalence, sensitivity, and specificity influence these values. Finally, the relationship between predictive values and sensitivity/specificity is compared, and the distinction between reliability and validity is clarified. Together, these concepts provide the foundation for selecting, interpreting, and applying diagnostic and screening tests in clinical and public health practice.