TROST, S.G. Objective measurement of physical activity in youth: current issues, future directions. Exerc. Sport Sci. Rev., Vol. 29, No. 1, pp 32-36, 2001. Second-generation activity monitors have revolutionized the way in which we measure youth physical activity. Use of the monitors avoids the problems associated with self-report methods and allows for the estimation of physical activity patterns over time. This article examines important methodological issues related to the use of activity monitors in children and adolescents.