peer reviewed ; Due to the continuous decrease of physical activity of children in industrialized countries physical activity behavior has become a key issue in health-related research. The purpose of this study was to assess objectively the daily physical activity of Luxembourgish children and adolescents and its distribution into school and leisure time. The physical activity behavior of 242 students (108 male students, 134 female students) with an average age of 12.84 years (SD = 2.37) was objectively assessed by wearing an accelerometer (ActiGraph GT3X-BT) for seven consecutive days. The average daily time spent in moderate to vigorous physical activity was 47.75 minutes (SD = 19.75). 62 students accumulated the recommended 60 minutes per day. 32% of students’ total moderate to vigorous physical activity occurred during school time and 63% during leisure time. During physical education, students were engaged in moderate to vigorous physical activity on average for 20.02%, whereas 46.93% of the time was spent being sedentary. Significant gender differences were found for all variables in favor of male students. Equally, physical activity in all areas decreased significantly with age. The Luxembourgish children and adolescents are insufficient physically active and the moderate to vigorous physical activity levels are mainly accounted by leisure time. Therefore, it seems reasonable for schools to create an environment that supports the physical activity throughout the school day to increase the daily physical activity, with special attention to female and older students.