This report has been commissioned by DFID, to strengthen the evidence base and inform policy development on debt sustainability in developing countries. It provides input to improve the Joint IMF-World Bank Debt Sustainability Framework for LICs, which will feed into the forthcoming review of the DSF. The paper begins by drawing lessons from past debt crises. They reflected universal over-optimism on commodity prices, ambitious spending plans including “high return” infrastructure projects, and access by relatively poor countries to market related funds. Past procedures for resolving debt crises were initially cumbersome, slow, fragmented and not based on considerations of debt sustainability. The paper then analyses what has changed since the crises, finding that in spite of important positive steps, the architecture for debt sustainability remains precarious. These trends have led to a rapid rise in debt in many countries. The paper analyses their sustainability, using thresholds from earlier work for DFID and endorsed by LIC policymakers. It finds that 22 of 28 DFID focus countries have unsustainable or borderline debt, with average levels unsustainable in Africa and Asia alike. The most negative trends are concentrated in countries widening their access to less concessional financing, and which are most “vulnerable”[…]