Abstract Health Impact Assessment (HIA) is often promoted as a tool to implement ‘health in all policies’ and sustainable development approaches to improve health and equity outcomes. However, is this truly the case? Currently global HIA practice is variable. This paper shares learning from the development and implementation of HIA in Wales to date to understand how HIA has been, and can be, used to inform policies and decisions and reduce health inequalities. The research investigated if, and how, HIA can be used as a tool to mobilise HiAP in reality. It followed a mixed methodological approach and investigated the contribution of HIA to practically advancing HiAP viewed through the lens of Wales and internationally. Primary and secondary evidence and statistics were collected and analysed. A literature review, stakeholder interviews, online statistical questionnaires and case studies were conducted. Thematic and content analysis techniques used. Findings include the importance of conceptual considerations i.e. the perspective on health - social determinant an equity or environmental determinant driven - taken by those carrying out or commissioning a HIA; the importance of centres of expertise to learn from; and practical considerations such as the political and regulatory environment, workforce and organisational knowledge and capacity. Wales has a dedicated expert HIA Unit, an enabling political environment and has led to HIA influencing policies and plans over 20 years. It has transferrable learning for other nations and the research including the online survey has been replicated by others, led to collaborations with nations such as Portugal and contributed to the reviewed literature by providing case studies on a topics like climate change which have influenced decision makers. Tangible ways forward for policy makers, researchers and academics include the need to develop more cross collaboration and fill gaps in the evidence i.e. on institutionalisation of HIA. Key messages • HIA is a excellent process to drive Health in all policies approaches. • HIA needs an enabling environment and workforce support such as that in Wales to thrive.