This paper describes the modelling exercise conducted for the Green Economy Report (GER) report and presents its results. The modelling was to test the hypothesis–which gave rise to the GER report–that investing in the environment delivers positive macroeconomic results, in addition to improving the environment. The paper also describes the key issues that need to be addressed by a modelling framework that tries to quantify the challenges of moving towards a Green Economy and the key features of the modelling structure.