This chapter provides background to the OECD Environmental Outlooks, explains the methodology used – including the traffic light system – and outlines the report structure. It focuses on the four Red light environmental challenges – climate change, biodiversity, water, and the health impacts of environmental pollution – identified as the most pressing challenges in the coming decades. The analysis in the Environmental Outlook is global, but its policy recommendations focus on OECD countries and the key emerging economies of Brazil, Russia, India, Indonesia, China and South Africa (the BRIICS). The Environmental Outlook makes future projections to analyse economic and environmental trends over the coming decades, by combining a general equilibrium economic modelling framework (the OECD’s ENV-Linkages model) with a comprehensive environmental modelling framework (the IMAGE suite of models of the PBL Netherlands Environmental Assessment Agency). The Environmental Outlook’s Baseline scenario presents stylised projections of what the world could look like in 2050 if current socio-economic and environmental trends and existing policies are maintained, but no new policies are introduced to protect the environment. To compare against the Baseline, the Outlook also presents the results of what if… simulations which model the potential effects of policies designed to tackle key environmental problems. This edition of the Outlook has been prepared for the Environment Ministers’ Meeting at the OECD in March 2012, and as an OECD input to the United Nations Conference on Sustainable Development (Rio+20) in June 2012. It is designed to be read alongside the OECD’s green growth strategy, Towards Green Growth.