In the past decade, citizen science has become a proven and accepted methodology across a wide range of scientific disciplines, able to collect new and complementary data which both enhances and adds context to existing data collection methods. By upscaling to a transnational level, citizen science could collect, analyse and exploit a vast amount of data across the ERA and beyond, achieving a higher impact through creating a multinational community of citizen scientists. However, many citizen science initiatives start at a small-scale, facing technical, practical and conceptual changes when attempting to upscale to a wider level, with current EU mechanisms and other networks not providing the support, coordination or resources required to assist their effort. CROPS aspires to inform and evolve the EU Research & Innovation system so that it can adequately support the transition of citizen science from small-scale to a Europe-wide level, moving it towards a modern, open-science approach. It will identify the most suitable citizen science initiatives for upscaling to the Europe-wide level, and in doing so will develop protocols, resources and examples of best practice for the upscaling of citizen science activities, helping practitioners to fully realise the potential impact of their activities towards the Horizon Europe EU Missions. This support will be tailored to all different types of citizen science, and the different stakeholders that are involved and participation taken.