Regular physical activity plays an important role in the normal development of children and provides beneficial effects on body composition, cardiovascular risk factors, bone health and mental well-being. Despite these benefits, epidemiological evidence indicates that activity levels decrease during the course of childhood and adolescence. This age-related decline of physical activity behavior is especially noticeable during early adolescence, a period in the lifetime of children that coincides with the transition from primary to secondary school. Accurate and detailed epidemiological information on the current activity levels of children is essential to fully understand the age-related decrease of physical activity behavior and to counter it through effective physical activity interventions and public health policy. The greater part of the studies that have examined physical activity patterns during childhood have used self-report techniques to quantify physical activity behavior. However, because of cognitive immaturity and intermittent activity patterns, children often experience difficulties remembering their past physical activity behavior. Consequently, retrospective self-report techniques are not the most appropriate tools to accurately assess physical activity behavior in children. In recent years, physical activity research in youth has evolved towards objective assessment using activity monitors. In the current research project two assessment techniques were combined (i.e. the SenseWear Mini Armband and an electronic activity diary) in order to obtain a more detailed and comprehensive image on patterns of physical activity and sedentary behavior of 10 to 14 year-old children. The project consists of two interrelated chapters regarding the objective assessment of physical activity and sedentary behavior in children and adolescents. The first chapter addresses methodological aspects of physical activity assessment in children using the SenseWear Mini Armband. In the second chapter, levels and patterns of physical activity and sedentary behavior are investigated in detail together with their relation with components of health-related fitness. The aim of the methodological chapter was to examine the validity of the SenseWear Mini Armband for the estimation of energy expenditure and to establish device-specific intensity thresholds for the SenseWear Mini Armband in children. The results indicated that the SenseWear Mini underestimated energy expenditure across the intensity spectrum. At the higher end of the intensity spectrum (i.e. very vigorous activities) a leveling off in energy expenditure estimation was observed. Given the high correlation between energy expenditure estimates of the SenseWear Mini Armband and the criterion, intensity thresholds were determined to discriminate between sedentary, light, moderate and vigorous intensity activities in 10 to 14 year-olds. The findings from this methodological paper were used to optimize the assessment accuracy of a 7-day physical activity monitoring protocol in the second chapter of the research project In the first paper of the epidemiological chapter, intensity- and domain-specific levels of physical activity and sedentary behavior were established in a sample of 10 to 14 year-olds. Moreover, differences in physical activity and sedentary behavior between week versus weekend days, genders and school stages were examined. The results indicated that activity levels were higher on weekdays compared with weekend days. In general, boys were more active than girls and primary school children more active than secondary school children. Nonetheless, differences in intensity- and domain-specific activity levels revealed meaningful nuances that are important for the development of physical activity interventions and promotion strategies. The second paper described and examined temporal patterns of physical activity and sedentary behavior on weekdays in our sample. The temporal pattern of physical activity and sedentary behavior was similar for boys and girls and for primary and secondary school children. The analysis indicated that at several time points during the day different types of physical activity and sedentary behavior go into competition with each other, most often in favor of sedentary pursuits. Moreover, the typical day schedule of children obstructs them to be active during the period immediately after school, also known as the ‘critical hours’ of physical activity behavior. By making adaptation to the typical day schedule of children, their daily activity levels might improve. In the final paper of this research project, the associations between objectively assessed components of physical activity and health-related fitness were investigated. The aim was to more precisely establish the link between physical activity and health-related physical fitness by addressing some of the constraints from earlier studies. In general, small-to-moderate associations were observed between components of physical activity behavior and health-related fitness. However, the associations between physical activity behavior and health-related fitness were stronger in girls compared with boys. Time spent sedentary and at light intensity activity was more strongly related to health-related fitness compared with moderate and vigorous physical activity. All together, the findings from the different papers highlight the importance of taking into account the complexity and multidimensionality of physical activity behavior 1) for the assessment for physical activity in field-based research, 2) for the description and interpretation of physical activity data and 3) for the development of physical activity interventions and promotion strategies in youth.