Tracking progress towards the United Nations’ Sustainable Development Goals (SDGs) requires high-quality, timely, and accessible data, often in areas where data are rarely available. Problems exist due to socioeconomic variations between countries and the qualitative nature of certain indicators in their definition. Citizen science has the potential to contribute to several SDGs. However, whilst citizen science’s potential to contribute towards SDGs is well documented, limitations exist when measuring the impact that citizen science has made toward SDG progress. To better understand the issues and prospective solutions surrounding impact assessment towards SDG progress, this work presents the outcomes of semi-structured interviews with citizen science project coordinators. They reveal the complex nature of impact assessment within a citizen science context. Coordinators demonstrate greater confidence when the project is easier to relate to the SDGs, and the project methodology can objectively measure indicators. Issues exist, however, when considering SDGs with a broader, global context, those more difficult to link to project goals and when the project’s impact on them happens at timescales beyond the funding period. If the full potential of citizen-science contributions to the SDGs is to be realised, approaches are needed to fully consider practitioners’ needs and motivations.

Evaluating the impacts of citizen science is challenging, and existing approaches often fail to capture the complexity of outcomes across domains. The MICS project (European Union Horizon 2020 Measuring the Impact of Citizen Science, 2019–2022) addressed this by developing a framework to measure the impacts of citizen science across five domains: society, science and technology, environment, economy, and governance. Central to MICS was the Impact Journey (IJ) Approach, a participatory evaluation framework combining domain-specific indicators with collaborative pathway mapping. This article critically reflects on the IJ Approach as both an evaluative tool and an intervention for co-creating impact with interested parties and actors (IPAs), based on its application in five environmental citizen science initiatives in Europe. To assess longer-term use of the IJ Approach in the cases, a post-project enquiry was conducted three years after completion of the MICS project. Analysis across cases showed the MICS IJ Approach effectively captured the complexity of citizen science impacts, visualizing causal links between assumptions, strategies, outcomes, and impacts. It revealed the diversity and context-dependence of impacts and the need for collaborative evaluation to prioritize them. However, its comprehensive and participatory method also entailed significant resource demands, constraining full implementation across all cases (further limited by disruptions from COVID-19). The article concludes by discussing practical challenges of participatory evaluation of citizen science initiatives.