Recent efforts have made progress in identifying ‘planetary boundaries’ and a ‘safe operating space’ in respect of human impacts on the environment and use of its natural resources. However, at the national level countries still lack robust and resonant metrics that allow them to make judgements as to whether they are operating within this ‘safe operating space’, nationally and locally, and whether therefore their economic activity is environmentally sustainable. This chapter presents the conceptual foundations of an index of environmental sustainability for nations, where environmental sustainability refers to the maintenance of important environmental functions. Target indicators representing environmental sustainability are identified across a whole range of policy-relevant environmental and resource issues. The distance from these targets to the current environmental performance can then be measured, and the information aggregated into a single index to show the extent to which countries are making progress towards environmental sustainability. An illustration of this procedure is given to show how such an indicator could give an overall easily comprehensible overview of progress over time towards environmental sustainability.