Most people are aware of the important influence that physical activity has on health outcomes and quality of life, and it has become increasingly important to be able to determine how much physical activity individuals participate in on a regular basis so that tracking of behavior change can occur and individual or population interventions can be implemented to modify behaviors and health outcomes. Many professionals, including teachers, administrators, nurses and recreational program leaders are interested in the amount of physical activity that takes place in their programs. Understanding methods of assessing physical activity behaviors in children and youth is important to those delivering physical activity-related programming. Assessing physical activity behaviors in children and adolescents is particularly problematic due to the nature of children's physical activity behaviors and their cognitive capacity to self-report their activity. This article describes the background and issues that often arise when measuring physical activity in youth, and provides recommendations for assessing physical activity behaviors in children and adolescents.