The development and maintenance of healthy physical activity behaviours from an early age is a priority for public health in the UK. Schools provide a number of different opportunities (time inside and outside of the curriculum) and resources in the form of space, equipment and staff for children to learn and develop healthy behaviours, at a time when they are most receptive to behaviour change. The overall aim of the thesis was to identify the different opportunities within the school context whereby children could be physically active and use theoretically driven, whole school interventions to optimise and subsequently increase healthful physical activity. Study 1 demonstrated that primary schools wishing to use pedometers within their curricula can be confident that the EZ-V model is sufficiently accurate to measure physical activity in the form of steps taken (r=0.897). Using the EZ-V pedometer, Study 2 demonstrated that feedback from pedometers along with information on how children can be physically active during the school day, can significantly increase children's mean daily steps-min"' compared to feedback alone or control groups over the course of a school week. Furthermore, boys were significantly more active than girls across each treatment group. Study 3 explored the affect of the primary school travel plan (TP) on the moderate to vigorous physical activity (MVPA) of 7-11 year olds during the winter and summer season. In order to examine the impact of the TP, schools were separated into schools deemed to have an Established TP (implemented for at least 2 years) or `New' schools (i. e., had just drafted their TP and were in the first year of its implementation). Children in the New TP schools accumulated 7.24 (winter) and 24.11 (summer) more minutes of MVPA (5.2% and 15.66% respectively) throughout the day compared with those in the Established TP school children (F(1,35=) 0.955, p=0.207, d=0.33). Overall, children were more active during the summer by 7.81 minutes (F(I, 35) = 0.089, p=0.768, d=0.1). The final study examined the affect of a 12 week, multi-component, whole school intervention which aimed to increase children's school day MVPA. Following baseline physical activity measures via accelerometry, intervention components consisting of a Health Week, Playtime Pals and a Pedometer Challenge were delivered sequentially over the first 6 weeks. Subsequent accelerometer data were collected after each intervention was delivered at 2,4,6 and 12 weeks. Results showed that from baseline to follow-up, children increased their MVPA by 6.57 minutes during the school day, which according to the Q statistic was likely to be beneficial. Results from the Pedometer Challenge found that boy s' mean pedometer steps"day-' were significantly higher than girls' (F(I, 95)= 9.987, p=0.002, d=0.65) and overall, mean pedometer steps-day"' significantly increased from week one to week five (F(,, 93)= 5.845, p=0.018, d=0.24). When the lowest and highest active 50% groups were compared, children in the lowest active 50% group significantly increased their steps from week one to week five (F(l, 47)= 20.847, p=0.000, d=0.93), while the highest active 50% did not (F(1,47)= 0.000, p=0.990, d=0). Furthermore, boys in the highest active 50% group were found to accumulate significantly more steps than the girls, in the highest active 50% group (F(I, 46)= 14.701, p=0.000, d=0.81), while there was no significant difference between the boys' and girls' pedometer steps in the lowest 50% group (F(l, 46)= 0.456, p=0.503, d=0.14). The overall findings of the thesis suggest that schools can successfully optimise the different opportunities during the school day in order to increase children's physical activity, but that larger, controlled and longitudinally designed studies are needed to confirm cause and effect. Most importantly, these changes may have most impact in the least active boys and girls. Interventions such as this should therefore be targeted at the least active children to ensure that they benefit as much as possible from the opportunity to increase their daily physical activity.