Background: Current statistics indicate a concern for the mental health and psychological well-being of Tongan youth in New Zealand and Australia. This impacts on their social and educational functioning and drives ongoing inequalities. Such data pleads for research which identifies potential solutions that are effective for Tongan youth addressing the issue of their mental health. The current thesis aims to build knowledge about underlying factors that may contribute to the adverse outcomes of Tongan youth living in New Zealand and Australia. Compared to the Pacific Island and the general New Zealand youth, Tongan youth have had the highest rate of suicide; highest rate of suicidal thoughts and highest rate of suicide attempts in New Zealand. Tongan youth also have poor educational attainment and are over represented in the criminal justice system. Pressures around adapting to a host country with differing family and cultural values are proposed to contribute to the complexities of life for Tongan youth. In order to understand the well-being of young Tongans, it is essential to take account of the impact of migration on them and their families. This thesis utilised a socio-cultural lens of acculturation gaps to explore how family and school related factors contribute to the compromised outcomes of Tongan youth in New Zealand and Australia. Methods: This study consisted of one hundred Tongan mother-youth dyads from New Zealand and Australia. The study utilised standardised measures of acculturation and enculturation, youth problems, family and school-related factors and substance use. The questionnaires were available in both the Tongan and English language. Results: High rates of psychological problems were reported. Twenty two percent of youth were in the clinical category for internalising problems, 29% in the clinical category for externalising problems and 45% in the clinical category for total problems. New Zealand Tongan youth compared to Australian Tongan youth were significantly higher in the clinical category for externalising problems but significantly lower in the clinical category for total problems. Notably, youth reported low levels of substance use when compared to national data. There were significant differences between mothers and youth on measures of acculturation. Typically mothers reported higher levels of enculturation (i.e., retained Tongan cultural values) compared to youth while youth reported higher levels of acculturation (i.e., adopting host cultural values). Acculturation and enculturation gaps did not directly predict youth outcomes on either externalising or internalising problems. However, family conflict mediated the relationship between acculturation gap and youth outcomes (both internalising and externalising problems). Furthermore, low commitment to school moderated the mediation of family conflict on acculturation gap and externalising problems. Conclusion: The outcomes of New Zealand Tongan and Australian Tongan youth warrant concern and intervention. The implications of these findings inform and equip policy makers and service providers to educate and facilitate Tongan families and communities, to improve the health and well-being of Tongan youth in New Zealand and Australia.