Reducing the environmental impact of digital services and technologies must be a priority both in the operation of existing services and in the design of future digital infrastructures. This deliverable presents the development of an environmental impact assessment methodology and guidelines for digital Research Infrastructures (RIs) resulting from the GreenDIGIT project. The GreenDIGIT impact assessment methodology aims to assess the impact magnitude of RI practices related to carbon emissions, energy consumption, waste management, and water use across their lifecycle. The deliverable proposes a set of metrics and indicators, which RIs can use to assess their impact for these categories, including Global Warming Potential (GWP), Cumulative Energy Demand (CED), Waste of Electrical and Electronic Equipment (WEEE), and Resource Depletion: Water (RDW). The methodology provides a framework to define improvement and implementation priorities to help RIs address the most critical impacts as a priority and to implement actions in a resource-effective manner. To do so, the methodology provides three parameters to guide the assessment: impact magnitude, likelihood, and effort required. The methodology is complemented by a self-assessment questionnaire (reported in GreenDIGIT Deliverable D8.1), which offers a practical tool to complete the assessment steps and answer detailed questions on the capability level of each topic area. The methodology provides a theoretical base for the assessment, and the questionnaire offers a practical tool to implement it. They have been developed as a joint service for RIs to assess their overall environmental sustainability ex ante and formalise an action plan to monitor and report on progress ex post. The RI assessment has been tested and validated by the participating RIs of GreenDIGIT (EGI, EBRAINS, SoBigData, SLICES) with reports on the use cases outlined in the deliverable, offering customised environmental reports with recommended actions tailored to their assessment levels. The deliverable also offers a set of general practical guidelines intended to support RIs at each stage of the lifecycle from concept development to termination. These guidelines can be used either with the RI assessment or independently by providing in-depth recommendations that RIs can use for establishing best practices. [This deliverable is pending approval from the European Commission]