In this paper we present the International Wealth Index (IWI), the first comparable asset based index for household’s long-term economic status that can be used for all low and middle income countries. IWI is similar to the widely used wealth indices included in the Demographic and Health Surveys and UNICEF MICS surveys, but adds the property of comparability across place and time. A household’s ranking on IWI indicates to what extent the household possesses a basic set of assets, valued highly by people across the globe. IWI is based on data from 2.1 million households in 97 developing countries. Test analyses show IWI to be a reliable and valid index. National IWI values are highly correlated with established welfare measures like the Human Development Index (r=.90); IWI-based poverty measures are highly correlated with poverty headcount ratios. With IWI we provide a stable and understandable yardstick for evaluating and comparing the economic situation of households, social groups and societies across all regions of the developing world.