There is growing concern over the effects of sedentary lifestyles on the health of young people. Recent rapid increases in juvenile obesity have received a great deal of attention in the scientific and popular press and have been attributed partly to television viewing, computer games and other sedentary behaviours. These are thought to compete with physical activity. There is a 'moral panic' concerning the 'couch kids' culture in modern western society. Project STIL (Sedentary Teenagers and Inactive Lifestyles) at Loughborough University is investigating 'what young people do' and focuses on active and inactive pursuits chosen in their leisure time. The following issues are addressed in this paper with specific reference to young people: how do we define 'sedentary behaviour' and do key sedentary behaviours displace physical activity? Are key sedentary behaviours obesogenic? What are the secular trends for children and youth for TV viewing? Our results for young people suggest that: 1. TV viewing and video-game playing are largely uncorrelated with physical activity, suggesting that there is time for both 2. meta-analytic findings show that body fatness is not related in any clinically meaningful way with key sedentary behaviours 3. although more children and youth have greater access to TVs than in previous generations, the amount of TV watched per head has not changed for 40 years. Preliminary findings from Project STIL suggest that inactivity is more complex that we sometimes think. Indeed, measures of 'couch potato-ism', such as TV viewing, may be inappropriate markers of inactivity.