The World Health Organization is the agency responsible for reporting the Millennium Development Goal (MDG) indicator "percentage of population using solid fuels." In this article, we present the results of a comprehensive assessment of solid fuel use, conducted in 2005, and discuss the implications of our findings in the context of achieving the MDGs.For 93 countries, solid fuel use data were compiled from recent national censuses or household surveys. For the 36 countries where no data were available, the indicator was modeled. For 52 upper-middle or high-income countries, the indicator was assumed to be < 5%.According to our assessment, 52% of the world's population uses solid fuels. This percentage varies widely between countries and regions, ranging from 77%, 74%, and 74% in Sub-Saharan Africa, Southeast Asia, and the Western Pacific Region, respectively, to 36% in the Eastern Mediterranean Region, 16% in Latin America and the Caribbean and in Central and Eastern Europe. In most industrialized countries, solid fuel use falls to the < 5% mark.Although the "percentage of population using solid fuels" is classified as an indicator to measure progress towards MDG 7, reliance on traditional household energy practices has distinct implications for most of the MDGs, notably MDGs 4 and 5. There is an urgent need for development agendas to recognize the fundamental role that household energy plays in improving child and maternal health and fostering economic and social development.