Mental illness is a worldwide public health concern. Factors that protect against mental illness and enhance emotional wellbeing require investigation and . physical activity has been identified. as potentially beneficial. This research comprised three phases: a systematic review of studies investigating any relationship between physical activity and emotional wellbeing in young people; a qualitative study of young people's understanding of any relationship; and a cohort study to determine whether physical activity is a protective factor for emotional wellbeing in young people. Thirty six studies (8 RCT's or controlled intervention studies and 28 cohort studies) were identified as eligible for inclusion in the systematic review. The evidence had many weaknesses making it difficult to draw any clear conclusions about any relationship. A sample of young people from the AHEAD study schools (n=57) took part in focus groups and paired interviews (14-15 years). A thematic analysis revealed that physical activity, while seen as important, was only one of 'many factors identified as enhancing young people's wellbeing. A prospective cohort (n=673) was formed based on the participants of the AHEAD trial. Measures of physical activity and emotional wellbeing were taken at baseline, when participants were aged 12-13 years, and again three years later in the follow-up study (15-16 years). Accelerometry was used to measure physical activity (volume and MVP A) and emotional wellbeing was measured using the WEMWBS and SDQ. Multivariable regression analyses was used to investigate the association between physical activity and emotional wellbeing, controlling for potential confounders. There was only evidence of an association between physical activity volume and the SDQ emotional problems subscale (one of five subscales) (-0.11 (95% CI -0.23 to 0.00) p=0.04). The findings of these three methodologically varied studies provide no strong evidence that physical activity has a beneficial role, although small effects cannot be ruled out.