Developed countries have committed to raising $100 billion a year to help finance climate mitigation and adaptation activities in developing countries. Where will this money come from? The new report from Institute of Development Studies and Stamp Out Poverty looks at this question. It develops a tool-kit to enable interested parties to assess and compare the various funding proposals on the table. By applying this methodology, the report suggests a 'portfolio' of funding mechanisms that could generate in over $70 billion a year, helping developed countries to meet their pledges. Notably, a tax on financial transactions stands out amongst the other mechanisms in terms of the quantity of revenue that could be generated.