Physical activity participation is beneficial to children’s health. In comparison to the general population, children of low socio-economic status (SES) are at increased risk of reduced well-being. Despite many attempts to increase physical activity and well-being there remains a lack of theory-informed interventions that target children of low SES. Within this thesis, five research studies are reported. Each study contributed to current understanding of physical activity and well-being promotion for children of low SES using Self-Determination Theory (Deci & Ryan, 2000), a theory that is often cited but not comprehensively tested in school-based physical activity interventions. Study 1 presented a systematic review of SDT- based physical activity interventions in the school-setting, concluding that more studies were needed to determine the efficacy of SDT as a behaviour change framework. Future studies were advised to apply validated psychometric instruments alongside methodological guidance. Study 2 validated psychometric instruments assessing SDT constructs of wellbeing and needs satisfaction with children of low SES. The predictive validity of SDT was statistically confirmed in Study 3, as needs satisfaction predicted children’s physical activity levels and well-being. Study 4 was a longitudinal clustered randomised controlled trial called Sport for LIFE: All Island (SFL:AI). While null effects were reported for SFL:AI on physical activity, well-being and needs satisfaction, researchers were recommended to integrate physical activity within a school environment, train schoolteachers in SDT principles, and model SDT hypotheses when determining the intervention’s effects. The final study provided needs-supportive training to schoolteachers and student volunteers through the Healthy Choices Programme. The children’s physical activity levels and well-being were enhanced through improvements in autonomy-support, needs satisfaction and intrinsic motivation, supporting SDT hypotheses. Practical, methodological and theoretical contributions from all five studies include: evidence-based guidance for training teachers, instructors and coaches to adopt needs-supportive principles; rigorous methods in the design and analyses of school- based programmes; the testing and validation of SDT instruments; and; advancement of SDT application in the prediction and enhancement of children’s physical activity and well-being. In conclusion, this research demonstrated that children’s physical activity levels and wellbeing can be enhanced by engendering a needs-supportive physical activity context that is conducive to psychological needs satisfaction and intrinsic motivation. To advance health promotion efforts for children of low SES, researchers, practitioners and public health advocates may consider replicating the SDT principles in the design and evaluation of programmes as critically evaluated in this PhD.