In recent years, many research results indicate that adult cardiovascular disease risk factors and obesity are associated with physical activity during childhood and adolescence. Since evidence indicates that there is a decline in physical activity during childhood and adolescence, the physical activity of children and adolescents has been given more attention lately. However, it is a particular challenge to accurately assess physical activity in children and adolescents. The article is a review of the definition and methods for assessment of physical activity. It can be used as clinical references for health professionals to perform an assessment of physical activity among children and adolescents.