The WeObserve project hosted a special event, “Observing the Environment: Challenges and Opportunities in Citizen Science”, held on 9 Oct 2019 in Brussels in cooperation with the European Commission. It brought together four sister projects on Citizen Observatories (COs) for Environmental Monitoring, funded by H2020 and launched in 2016, and commission representatives working closely with citizen science initiatives. The participating CO projects (LandSense, GROW Observatory, Scent, and Ground Truth 2.0), which have either concluded their activities or are in their final year, showcased their achievements and shared best practices. The focus of the event was an exploration of the challenges facing COs to generate forward-looking solutions and recommendations that can help improve sustainability and increase impact for ongoing and future COs. The main outcomes and insights from the event are documented in this report.