The present commentary provides a brief overview of and reflections on the joint findings of two reviews of the present evidence regarding correlates, predictors and/or determinants of sedentary behavior among youth and older adults published in the International Journal of Behavioral Nutrition & Physical Activity.In both reviews, the included studies were predominantly conducted in Europe, the US, and Australia. Most studies were limited to TV or ‘screen’ time rather than sedentary behavior and relied on self-report. In both age groups there is a lack of qualitative studies as well as studies looking into the more motivational and contextual potential determinants of sedentary behavior. Both reviews indicate that to date there is limited evidence on the determinants of sedentary behaviour in youth and older adults. In youth, age and weight status were identified as determinants of sedentary behavior, with more sedentary time among older and heavier kids. In adults, age and retirement were determinants, with older and retired elderly sitting more.