The Perceived Stress Scale (PSS) was developed as a self-report, subjective measure to evaluate the extent to which situations are assessed as stressful in an individual’s life. The PSS determines the extent to which respondents found their lives overloaded, uncontrollable, and unpredictable. Since its inception, the PSS has been widely adopted within both general and clinical settings receiving over 20,000 citations on Google Scholar, and the scale has been translated into 28 different languages. This chapter presents a theoretical overview of stress, the PSS scale’s development and initial validation, further examination of the psychometric properties from more recent studies, and a brief outline of the three versions of the PSS, with the main focus of the chapter on the most commonly used scale, PSS-10. The PSS has shown to be a reliable measure of perceived stress with robust psychometric properties that are suitable for mindfulness research.