This guidance manual explains what Health Impact Assessment (HIA) is and the stages involved in conducting it. It has been revised and updated based on the experience of HIA practitioners and includes new tools which have been developed to assist each step of the HIA process. It aims to provide a user friendly and practical framework to guide policy-makers and practitioners in undertaking HIA. All HIA tools contained in this guidance and further information on HIA may be found at https://publichealth.ie/hia-guidance/