The growing urbanization requires that cities will have to increase their participation in natural ecosystems. Today, it is generally accepted that Nature-Based Solutions will play an important role in cities regeneration, since they address urban challenges in a multifunctional way. In fact, green spaces provide multiple benefits both for people and nature. Green roofs and green walls are some of the most promising alternatives as urban space is scarce and they can green the grey infrastructure. Yet, their merit is not generally known and accepted, mainly due to difficulties in valuing intangible benefits as biodiversity. Biodiversity guarantees an important amount of ecosystem services, such as food supply, soil composition, air quality or climate regulation. Therefore, public policies have pursued answers to protect biodiversity, namely on the basis of international agreements, such as the United Nations Sustainable Development Goals. However, despite the growing concern, the loss of biodiversity has continued, particularly in urban areas. The purpose of GRAVITY is to enhance the knowledge on green roofs/walls contribution to the biodiversity of cities