This book presents the results of the second international MasterClass hosted by Metrolab in January and February of 2019, on the theme of urban ecosystems in Brussels. The event was the outcome of a transdisciplinary methodological reflexion on the relations between urban ecologies and urban policies. How can urban ecosystems be approached from a transdisciplinary perspective? How can urban projects such as ERDF be mobilized to initiate a socio-ecological transition? Before delving into these methodological questions, we would first like to put this project back into the Metrolab context.