MICS is a Citizen Science (CS) impact measurement project funded by the European Commission under Horizon 2020. The project involves six European partners from five countries. MICS adopts and adapts the best practices generated by the Ground Truth 2.0 project in co-creating validated CS in four case studies in Europe, aiming at creating a comprehensive conceptual framework and formulating clear recommendations for those involved in CS projects. The four areas (from the UK, Italy, Hungary and Romania) explore CS co-creation in regions with different needs, contexts and approaches for managing environmental quality issues (for example, river restoration and implementation of green solutions – Nature Based Solutions) and different levels of CS application.