The existence of FAIR-aligned and harmonised data policies across various stakeholders such as funding bodies, publishers and Research Performing Organisations (RPOs) is crucial for ensuring that we can progress from a vision of the European Open Science Cloud (EOSC) to it becoming a fully functioning reality. As noted in the Turning FAIR into Reality report and action plan, policies define and regulate various components of a FAIR ecosystem and the relationships between them. Indeed, policies are a cross-cutting theme in Turning FAIR into Reality (TFiR) and are reflected in many of the priority and supporting actions presented in the action plan. This guide aims to help Research Performing Organisations to assess the data policy framework currently in place and to consider where possible improvements may be needed. To complement the guide, a FAIRsFAIR policy support checklist is available. This aims to help RPOs to consider the content of their data policy, and how they might better align it with the FAIR Principles. ACME-FAIR is a 7-part guide developed in the FAIRsFAIR project, whose main purpose is to help managers of Research Data Management and related professional services to self-assess how they are enabling researchers, and the professional staff who support them, to put the FAIR data principles into practice (for short we refer to this as ���FAIR-enabling practice���). This part addresses the key issue of Defining the FAIR data policy environment.