Urban agriculture (UA) is expanding in European cities and it has recently colonized the roof of buildings in the form of rooftop agriculture (RA), particularly due to urban land constraints. The implementation of RA has been done in multiple ways: from high-tech business-oriented solutions, often promoted by research and innovation programs of the European Commission, to agro-ecological and socially oriented initiatives lead by the citizenship. This paper evaluates the current state of RA implementation in Western Europe by reviewing existing projects. A systematic search through internet, UA-related networks and societies was performed to list the existing projects around Europe. A total of 33 initiatives were evaluated, including projects (concept and planned), implemented cases and already closed experiences. Four categories of RA project were revealed in Western Europe: gardens, farms, engineering and landscape. The multi-functionality of RA implies a combination of objectives and benefits, namely food and non-food production, economic and social services, financial and environmental impacts. The analysis also unveiled some implementation barriers that constrain the development of RA, limiting the long-term viability of existing projects. How many will remain in 5 years' time? And how many will replace them, and find a sustainable system? This is the role of research teams to study these initiatives, to test technical options and policy regulations through socioeconomic models and scenarios to offer an enabling environment for RA to become one way for cities to cope with climate change, food and nutrition security, biodiversity management and human prosperity.