ITU‐T's Technical Reports and Specifications 821 Key performance indicators definitions for smart sustainable cities Executive summary This Technical Report is a deliverable of the ITU‐T Focus Group on Smart Sustainable Cities (FG‐SSC) and is part of a series of Technical Reports and Technical Specifications focusing on key performance indicators (KPIs) for smart sustainable cities (SSC). This Technical Report is intended to supplement and provide further background on the content provided in the “Technical Specifications on overview of key performance indicators in smart sustainable cities”, which was approved during the FG‐SSC meeting held in Geneva, October 2014. It is also intended to complement the “Technical Specifications on key performance indicators related to the use of information and communication technology in smart sustainable cities” and the “Technical Specifications on key performance indicators related to the sustainability impacts of information and communication technology in smart sustainable cities”. This Technical Report provides a comparative analysis of nineteen different index sets. Supplementary information on each of the approaches reviewed is presented in the annexes, thus providing a comprehensive background of the resources that formed the FG‐SSC series of KPIs Technical Reports and Technical Specifications. In order to ensure the inclusion of a wide array of perspectives, the indexes reviewed originate from international sources, national/regional sources, city organization sources, academic sources, and company sources. By doing so, the analysis evidenced the broad set of perspectives and approaches used to measure and assess the performance of SSC, and in particular, the role of ICTs in urban sustainability, thus demonstrating the importance of KPIs development for smart sustainable cities. This Technical Report is structured around four main sections. Section one provides the introductory background and scope. Section 3 presents a series of key definitions used as the basis for this Technical Report. Section 5 presents the comparative analysis of indicators, structured around the five dimensions that characterize SSC strategies (i.e. ICT, environmental sustainability, productivity, quality of life, equity and social inclusion, and physical infrastructure). Based on this analysis, section 6 offers reflections and concluding remarks. The comparative review presented in this Technical Report provides a valuable background for the discussions held by the members of ITU's FG‐SSC, particularly for the exchanges that took place between the members of Working Group 3 focused on KPIs and metrics. It also helps to understand the development of the series of KPIs Technical Reports and Technical Specifications, confirming ITU‐T FG‐SSC commitment to the development of robust metrics that can serve as the basis for the development of standards in this field.