This deliverable provides CROPS’ Guidelines for Stakeholder Engagement to support citizen science projects not only to design and implement stakeholder engagement strategies, but also to plan strategies ambitioning to upscale into broader limits. The deliverable is a result of Task 3.3 (WP3) and translates the evidence collected from multiple sources (a structured literature review (70+ sources) and CROPS empirical inputs (project mapping, two surveys and expert interviews)) into practical, context-adaptable guidance for citizen science practitioners and project coordinators. The guidelines are organised as a 5+1 step process—identify stakeholders; map and prioritise them; design engagement strategies by stakeholder group; adapt engagement for upscaling; communicate and build trust; and monitor, evaluate, and adapt—recognising that these steps are sequential in logic but iterative in practice. The guidance is structured around the quadruple helix (citizens/civil society, academia, public authorities/policymakers, and industry/SMEs) and provides tailored approaches for each group, with cross-cutting attention to inclusivity, ethics and Responsible Research and Innovation. It highlights persistent gaps observed in practice (especially inconsistent policy engagement and rare, often funding-limited industry involvement) and proposes realistic ways to build coalitions, cultivate champions, protect open data and integrity, and sustain participation beyond initial recruitment. A dedicated upscaling section addresses scaling out, up, deep and down; the growing importance of intermediaries and coordination infrastructure; and the standardisation–adaptation tension (“standardise what must be comparable, localise what must resonate”). The deliverable concludes with targeted recommendations for practitioners, funders, policymakers and researchers, and annexes with ready-to-use checklists and templates to operationalise stakeholder planning, communication and evaluation.