It is the aim of this thesis to investigate Health Impact Assessment (HIA) use in public policy formulation in Northern Ireland and in the Republic of Ireland. The influences affecting the use of HIAs will be examined in this study. Four case studies, where HIA has been conducted, will be used for research analysis. This includes HIAs conducted on traffic and transport in Dublin, Traveller accommodation in Donegal, a draft air quality action plan in Belfast and on a social housing regeneration project in Derry. HIA aims to identify possible intended and unintended consequences that a project, policy or programme will have on the affected population’s health. Although it has been acknowledged as a worthwhile tool to inform decision-makers, the extent to which it is used in policy in Ireland is subject to scrutiny. A theoretical framework, drawing from institutionalist, impact assessment and knowledge utilisation theories and schools of literature, underpin this study. The investigation involves an examination of the unit of analysis which consists of the HIA steering groups. These are made up of local authority decision makers, statutory health practitioners and community representatives. The overarching structure and underlying values which are hypothesized as present in each HIA case are investigated in this research. Chapters 2 and 3 outline the main literature in the area which includes theories from the public health and health promotion paradigm, the policy sciences and impact assessment techniques. Chapter 4 describes the methodology in this research which is a multiple case study design. This is followed by an analysis of the cases and then concludes with practical recommendations for HIA in Ireland and theoretical conclusions of the research