The Sustainable Society Index measures the three fundamental pillars of sustainability (the economy, the environment, and development) for154 countries around the world. It assigns the same weighting to all the indicators, without any aggregation of the pillars. This study proposes the use of cross-efficiency in order to overcome these shortcomings and obtain a more accurate sustainability index that allows countries to be ranked in terms of their environmental situation as well as their economic and social development. First, cluster analysis is used to classify the countries into homogeneous groups, according to their environmental position. Then, two sustainability indices are produced to measure environmental as well as economic and social aspects. The results show that few countries have managed to improve all facets of sustainability, and at times economic development is associated with both social progress and environmental deterioration, which diminishes the end result.