It is an accepted convention that non-health sector policies and strategies impact on population health. An instrument and approach, Health Impact Assessment (HIA), seeks to assess the health impacts of projects, programmes and policies in a systematic way. The ultimate goal of HIA is to inform public policy processes of these impacts. This book provides for the first time an analysis of how and why HIAs informed local policy development in both jurisdictions on the island of Ireland. An original theoretical framework was used as the analytical lens for this exploration, drawing from the fields of political and social sciences, and public health. The HIA projects were conducted on traffic and transport, Traveller accommodation, urban redevelopment and air quality. This conceptually-grounded guide draws from the disciplines of the political and social sciences and public health, and will appeal to academics, students and practitioners in these fields as well as policy-makers and planners at local and national government levels.