Evidence suggests that sedentary behaviour (SB) is associated with adverse health outcomes. Children’s SB is a complex set of behaviours that includes different types of activities taking place in a variety of settings. Therefore, assessing children’s SB is challenging and currently no single method exists that captures the behaviour as a whole. This thesis aims to explore and develop new and existing methods of assessing children’s SB, by employing a range of quantitative and qualitative methods. Accelerometry has become a widely used method of estimating sedentary time (ST). Study 1 identified raw acceleration thresholds to classify children’s sedentary and stationary behaviours, using two accelerometer brands across three placements. Thresholds however, do not account for the postural element of SB, as per its definition. Study 2 validated the Sedentary Sphere method in children, allowing for the most likely posture classification from wrist-worn accelerometers. Study 3 added contextual information to accelerometer data by using a digitalised data capturing tool, the Digitising Children’s Data Collection (DCDC) for Health application (app). Children used the app to report their SBs daily through photos, drawings, voice recordings as well as answering a multiple-choice questionnaire. Results from the DCDC app identified specific SBs to be targeted in future interventions. Data showed distinct differences between boys and girls’ screen-based behaviours, suggesting gender-specific interventions are needed to reduce screen time. Using the DCDC app in combination with accelerometry often explained patterns of SB and physical activity observed in accelerometer data. Study 4 added information about parents’ perceptions of the factors that influence their children’s SBs. This study identified parents/carers as a target for future interventions in view of perceptions reported about PA and SB and their need for support to help reduce the time children spend using screen-based devices
