Globally, physical inactivity is the fourth leading modifiable risk factor for premature mortality. Adolescence is a critical time where physical activity (PA) habits developed may persist into adulthood. However, evidence demonstrates that PA levels decline with increasing age in adolescence. The purpose of this thesis is: (1) to validate a PA instrument for use in youth aged 10 – 18 years; (2) to appraise tracking of PA and sports participation over a 5-year period in youth; and (3) to assess behavioural and psychosocial factors that influence PA over time among Irish children and youth. As part of the Children’s Sport Participation and Physical Activity study, data were collected on a sample of 5,397 youth aged 10 – 18 years (46.6% male) in the Republic of Ireland. Five years later, participants (n = 873; 30.4% male) completed multi-section selfreport surveys developed using Social Cognitive Theory and Ecological Theory as part of a longitudinal study. All elements included in the surveys were psychometrically valid and developmentally appropriate measures that have been validated for use in adolescents. Four papers were undertaken as part of this thesis. In paper 1, levels of subjectively reported PA from the survey were validated with objective measurement of activity (via accelerometry), in a sub-sample of participants. In paper 2, changes in PA levels and sports participation were evaluated between time point one (2009) and two (2014). Results suggest that PA can, to an extent, track into later life and advocate for a greater frequency of sports participation and competitive sports engagement in youth. Paper 3 assessed PA domains (club and extra-curricular sports participation, active commuting to school and physical education) as predictors of 5-year PA. Of the PA domains examined, sport plays the most valuable role in sustaining PA involvement. A description of the uptake, dropout and participation levels in different sports and reasons for a change in participation are included in paper 3. In paper 4, the psychosocial determinants of youth PA were appraised using the framework of the Youth Physical Activity Promotion Model. Findings indicate that friend social support and PA self-efficacy play instrumental roles in influencing 5-year PA in youth. PA promotion strategies should include frequent, high-quality opportunities for sports participation, consider what sports are attractive to young people, and address the reasons for uptake and dropout identified in this thesis. Efforts to promote longevity in PA behaviour should direct attention to improving self-efficacy for PA and encouraging peer social support.