From its inception as a policy goal, advocates of sustainable development have emphasized the need to measure progress using suitable sustainability indicators. Even the early rhetoric on sustainable development recognized the importance of measuring progress and sustainable development indicators (SDIs) have long been seen as one of the essential tools for implementation. Indeed, following the 1992 Rio Summit on Environment and Development, countries were urged to ‘develop indicators of sustainable development’ in a way that would ‘contribute to a self-regulating sustainability of integrated environment and development systems’ (UNGA, 1992: para. 40.4).