This chapter explores the significance and application of patient-reported outcome measures (PROs) in medical practice, particularly in evaluating the effectiveness of healthcare interventions. The use of PROs provides direct insights from patients about their health status, facilitating a comprehensive understanding of treatment impacts on symptoms and quality of life. The chapter outlines the historical development of measurement tools, and emphasizes the evolution from classic to modern test theories. The chapter covers the psychometric properties of PROs, such as reliability, validity, and responsiveness, and highlights the necessity of proper measurement tools in clinical trials to derive meaningful conclusions. Through detailed explanations, the chapter addresses the challenges in creating valid and reliable PROs and the implications of using flawed measurement tools in research. The chapter concludes by with a discussion on future directions, and advocates for the continued development of robust, patient-centered PROs and the implementation of the consensus-based standards for the selection of health status measurement instruments (COSMIN) guidelines to ensure high-quality measurement tools.