It is well established that physical activity is associated with significant physical and mental health benefits including increased longevity. These data have prompted an increased interest in promoting physical activity, which requires accurate and objective quantification of activity. The main objectives of this chapter are to (a) review the epidemiologic evidence documenting the beneficial health effects of physical activity; (b) define physical activity and related constructs; (c) present an overview of commonly used laboratory and field measures of physical activity that are relevant to researchers and clinicians in behavioral medicine; and (d) discuss the emergence of new technologies that may improve the assessment of physical activity. Depending on the physical activity parameter of interest (e.g., frequency, intensity, duration, energy expenditure), participant population, sample size, data sampling frequency, and study location and duration, researchers and clinicians have many assessment instruments to choose from. Laboratory, field, self-report, and physiological measures are each associated with unique costs and benefits; thus, instrument selection should be tailored to the aims of the assessment.