High amounts of time spent sedentary and low levels of physical activity have been implicated in the process of excessive adiposity gains in youth. The aim of this review is to discuss the role of physical activity, sedentary time and behaviour (i.e. television (TV)-viewing) in relation to adiposity during the first two decades of life with a specific focus on whether the association between sedentary time, and behaviour and adiposity is independent of physical activity. We identified nine cohort studies (three prospective) whether sedentary time was associated with adiposity independent of physical activity. Eight of these studies suggested that sedentary time was unrelated to adiposity when physical activity was taken into account. Results from studies (n 8) examining the independent association between TV-viewing and adiposity independent of physical activity were mixed. Those that observed a positive association between TV-viewing and adiposity independent of physical activity discussed that the association may be due to residual confounding. A few additional studies have also challenged the general notion that low levels of physical activity leads to fatness and suggested that higher baseline fatness may be predictive of a decline in physical activity. It appears unlikely that higher levels of sedentary time are associated with or predictive of, higher levels of adiposity when physical activity is controlled for in youth. Specific sedentary behaviours such as TV-viewing may be associated with adiposity independent of physical activity but the results may be explained by residual confounding.