Twenty years after the first Earth Summit held in Rio de Janeiro, UNEP launched the Global Initiative for Resource Efficient Cities (GI-REC), with the goal of applying integrated approaches and analyses such as urban metabolism in city planning and management. GI-REC was tasked with developing practical applications of academic concepts highlighted by the IRP’s reports. This meant bringing together both scientists and policy makers, and ensuring that their different worlds and priorities connected. Through these various science-policy collaborations, tools were developed and piloted by GI-REC in several cities around the world. This summary report describes the original work that has been developed under GI-REC. It comes at an opportune time, when cities are at the forefront of global environmental discussions. In addition to bringing together professionals of different disciplines, GI-REC also brought together two separate work streams of climate and resource efficiency. By looking at cities from a systems perspective, the Initiative provided guidance on the transition of cities from a linear to a circular economy, and on alternatives to the way our cities are being planned and built.