With the fresh impetus for sustainable development and green growth highlighted in this Development Co-operation Report, it is essential that we have robust and credible methods for measuring the financial resources pledged and allocated to achieve these goals. This chapter, contributed by the statistics team in the OECD’s Development Assistance Committee, explains how aid to sustainable development is measured, how it has grown over the years and what challenges remain. The current marker system has already helped to raise awareness in donor agencies of the importance of mainstreaming environment in all development co-operation as demonstrated in the statistics presented elsewhere in this report. Future efforts are called upon to facilitate tracking international official and private flows in support of the Rio conventions and environmental objectives more generally.