‘Leaving no one behind’ (LNOB) constitutes one of the core principles underpinning the 2030 Agenda for Sustainable Development. In this paper, we propose a fuzzy logic approach to identify countries left behind in each dimension of human development and to estimate the extent to which certain countries are left behind in terms of overall human development. Following the current analytical framework for measuring the Human Development Index (HDI), we illustrate our proposal by measuring the degree to which a country was left behind in the years 2000 and 2018. In general, we find that the countries left furthest behind at the beginning of the century were those that most reduced gaps with respect to better performing countries. Nevertheless, we cannot clearly speak of convergence in HDI as there are notable exceptions, such as the Central African Republic, Liberia, Yemen, Haiti, and Venezuela, which despite the improvement in their HDI between 2000 and 2018, worryingly increased their gaps in human development relative to the rest of the world. The illustration highlights the significant advantages of measuring crosscounty human development using our fuzzy-based LNOB approach to provide new complementary measures consistent with the United Nations’ moral imperative of leaving no country behind.