Aims: to investigate: 1) whether the magnitude of change in sedentary time and physical activity during adolescence varies in different time contexts, 2) whether the estimate of change in sedentary time differs according to the measurement instrument used and 3) whether self-reported sedentary activities are associated with sedentary time. Methods: In Study 1, changes in sedentary time and physical activity between the ages of 12 and 15 years were assessed using data from the PEACH project. Sedentary time and physical activity were measured using ActiGraph accelerometers (GT1M). To determine if the magnitude of change differed for during-school, after-school and at weekends random effects mixed models were used. For studies 2 and 3, 124 adolescents wore an activPAL and an ActiGraph (GT3X) accelerometer for 5 days. For Study 2, direct comparisons between the activP AL and the ActiGraph estimates of sedentary time were made using Bland-Altman plots. For Study 3, differences in activPAL-estimated sedentary time between younger and older adolescence during-school, after-school and at weekends were assessed and the association between objectively measured sedentary time and self-reported sedentary activities were assessed. Parallel analyses were carried out using ActiGraph-estimated sedentary time. Results: The results of Study 1 showed that the proportion of time spent sedentary increased between the ages o~ 12 and 15 years and that this increase occurred during-school, afterschool and at weekends (+8.2%, (95% CI = 7.4 to 9.1), +7.0%, (95% CI = 5.9 to 817), +6.9%, (95% CI = 5.1 to 8.6), respectively) and a concurrent decrease was shown in light intensity physical activity (-7.6% (-8.3 to -7.0), -7.0% (95% CI=-7.7 to -6.3) -6.7 (-7.8 to -5.7) respectively) and minimal changes in moderate to vigorous physical activity. There was no evidence that the magnitude of these changes differed between time windows. Study 2 showed that ActiGraph estimated sedentary time was 6.9 minutes an hour higher than the activPAL estimated sedentary time and there was a large degree of variability in the agreement (95% LoA=-2.3 to 16.2). Moreover, in Study 3 differential results were shown in the association between objectively measured sedentary time and self-reported sedentary activities and in the cross-sectional differences in sedentary time between younger and older adolescents depending on which monitor was used to measure sedentary time. For example, the activPAL found a difference of +11.4%,+11.6% and +3.0% in for during-school after-school and weekends whereas the differences in ActiGraph estimated sedentary time was +6.2%, +5.1 % and +4.7% respectively. Conclusions: Interventions to attenuate the increase in sedentary time between the ages of 12 and 15 years are required during-school, afterschool and possibly at weekends. Moreover, there are differences in the activPAL and ActiGraph estimates of sedentary time. If a more precise estimate of sedentary time is required, it may be that the activPAL compared to the ActiGraph may be the more appropriate measure to use.