According to the most recent (2011) global census of eco-city initiatives, there are currently 178 eco-city initiatives under development, representing a significant mainstreaming of urban sustainability in the last decade. As the number of eco-city initiatives grows, so the question of how to define eco-city indicators and establish standards becomes more pressing. While there are many sustainability standards and certification schemes available for use at building level (e.g. LEED, BREEAM), similar sustainability assessment and endorsement frameworks for the urban level have only recently begun to emerge. This article surveys the current situation by: (i) proposing a conceptual model of urban sustainability indicators from a governance perspective; (ii) presenting the findings of a comparative analysis of the use of urban sustainability indicators in nine eco-city initiatives; and (iii) outlining key challenges for the future development of international urban sustainability standards. It argues that the current situation is marked by a considerable diversity of practice and governance functions, and an ongoing tension between place-specificity and universal applicability as goals of urban sustainability.