The Sustainable Development Goals (SDGs) require major societal transformations that will depend on significant fiscal outlays and private investments. These fiscal outlays cover public investments, the public provision of social services, and social protection for vulnerable populations. The key message of this report, in line with and building on recent findings from the IMF Fiscal Affairs Department (FAD), is that the governments of the Low-Income Developing Countries (LIDCs), a group of 59 low-income countries with annual incomes below US $2,700 per capita, will have to increase budget outlays significantly to achieve the SDGs, significantly outstripping their current and potential domestic revenues. It examines a three-prong approach for closing the SDG budget gap it identifies: 1) increased domestic revenues; 2) increased Official Development Assistance (ODA) to governments; and 3) increased Private Development Assistance (PDA) to governments. The estimation of this report is that the SDG public financing gap for the world’s poorest countries is on the order of $300-400 billion USD, an amount equal to a small share of the incomes of donor countries and the world’s wealthiest individuals. This report also emphasizes that precise estimates of SDG financing needs depend on further, country-specific costing analyses that LIDC governments and the international community should undertake as a matter of priority.