Technical Abstract: Physical activity is an important component of total energy expenditure, contributing to energy intake needs; it also provides certain health benefits. This review chapter provides state-of-the-art information to researchers and clinicians who are interested in developing research studies or interventions with physical activity components. The topics covered include important physical activity assessment issues, relevant aspects to be considered when selecting a method for assessing physical activity, and details about the different methods available, including new methodological advancements.