Health Impact Assessments (HIA), at its core, seeks to identify who is going to be affected in the population by a policy or plan and how it will impact their wider health and wellbeing. This chapter primarily focuses on an HIA conducted in Wales, which centred around the health, well-being, and equity impacts of climate change. It describes the process and results of the HIA and highlights how HIA is a useful tool for policymakers considering action on climate change. The chapter briefly outlines the quantitative aspects while also emphasizing the benefits of qualitative evidence within the process—for example, stakeholder engagement and interviews with key informants from a range of national organizations. Finally, the chapter illustrates how an HIA can help identify the differential effects of climate change on the health and well-being of diverse populations.