This thesis explores issues relating to intervention in, and measurement of, child mental health and well-being. The first chapter was a systematic review of Positive psychology interventions used to improve mental health and well-being outcomes. This identified 12 randomised controlled trials (n= 1668) in children and adolescents identifying a growing but generally weak evidence base. The array of outcomes and confounding variables assessed were identified alongside implementation issues. The second chapter explored psychometric properties of a ubiquitous tool for measuring these same outcomes, namely the Strengths and Difficulties Questionnaire. Factor structure, reliability, and measurement invariance were investigated and normative data developed. A 5-factor model was found to best fit the data in a large nationally representative sample. Omega (but not alpha) reliability coefficients revealed acceptable reliability and measurement invariance (configural, metric, scalar) was found for gender, parent status, and socio-economic status. Findings suggested that the method of analysis employed greatly affected subsequent interpretation. Normative analysis revealed current norms underestimate at risk children (by 1-2 points) and new norms were created. The clinical, theoretical, and research implications of these findings are presented in the final part of the thesis, alongside a series of recommendations.