The provision and administration of high quality urban public green spaces intertwines issues of planning, design, management and maintenance with governance. The benefits of such spaces are often tied to social justice, public health and recreation, biodiversity and helping cities to deal with climate change. International policies and changes in public administration have encouraged user participation across multiple phases of green space development. Although sceptics towards participation are easily found supporting arguments sometimes stand without critique, not questioning how participation affects the physical quality of green spaces. This literature review surveyed empirical scientific studies seeking to answer the following research question: How does research to date reflect over user participation's contribution to public urban green space quality? The review includes 31 articles from peer-reviewed scientific journals and finds an array of arguments used to support and attribute potential benefits to participation. However, analysing what has been empirically tested in these articles shows an even and general lack of proof for these arguments, implying that many arguments for participation are taken for granted. A particularly large disparity was found between the discussing and testing of many arguments regarding how participation may directly benefit urban green spaces. Rather than assessing the physical outputs of participation, most of the empirical studies tested process benefits to users and administrators. Due to the discovered predominance of these process-driven studies, it remains unclear whether participation actually improves green spaces, or if it is just for the benefit of the people involved. The gap in scientific knowledge found here calls for a re-focus to case level research, empirically testing where the actual benefits of participation lie and how participation processes might best lead to high quality green spaces in practice.