Considering the costs and risks of inaction, ambitious action to reduce greenhouse gas emissions is economically rational. However, success in abating world emissions will ultimately require a least-cost set of policy instruments that is applied as widely as possible across all emission sources (countries, sectors and greenhouse gases). The main purpose of this paper is to explore feasible ways to meet these two basic requirements for successful future climate policies. Using a range of modelling frameworks, it analyses cost-effective policy mixes to reduce emissions, the implications of incomplete coverage of policies for the costs of mitigation action and carbon leakage, the role of technology-support policies in lowering future emissions and policy costs, as well as the incentives –and possible options to enhance them – for emitting countries to take action against climate change.