Addressing complex global health challenges, including the burden of noncommunicable diseases (NCDs), will require change in sectors outside of traditional public health. Contemporary regional trade and investment agreements (RTAs) like the Trans-Pacific Partnership (TPP) continue to move further ‘behind-the-border’ into domestic policy space introducing new challenges in the regulation of health risk factors. This dissertation aimed to clarify the pathways through which RTAs influence NCDs, and to explore points along those pathways with the intent of improving the existing evidence base and supporting policy development. This work develops a critical theoretical framework exploring the ideas, institutions, and interests behind trade and investment policy; it also develops a conceptual framework specifying how trade and investment treaty provisions influence NCD rates through the effects of trade and investment on tobacco, alcohol, and ultra-processed food and beverage products, as well as access to medicines and the social determinants of health. Using health impact assessment methodology, three analytical components were designed to examine pathways of influence from RTAs to health outcomes as mediated by the interests of transnational corporations (TNCs). The first component explored the influence of industry during the TPP negotiations and how its health-related interests were reflected in the final TPP text. The second component examined the role of trade and investment liberalisation in health-harmful commodity markets, finding a rise in TNC sales after a period of liberalisation. The third component demonstrated how investor rights and investor-state dispute can challenge the state’s right to regulate if it damages the profits of TNCs, which may threaten effective health regulation, and provides opportunities to strengthen the right to regulate. The work in this dissertation provides support for the thesis that trade and investment policies are a fundamental structural determinant of health and well-being, which are highly influenced by TNCs that guide such policies in the interest of maximising their profits and protections, often to the detriment of public policy and population health. This work identifies the need for more robust health impact assessments of RTAs before future agreements are ratified, as well as an imperative to challenge vested interests that entrench neoliberal policy preferences that have hindered sustainable and equitable development.