This Supplement provides a general overview of a key set of indicators related to the use of information and communication technology (ICT) and corresponding impacts on city sustainability in smart sustainable cities (SSC). It is intended for an audience of SSC decision-makers and strategists, interested in gaining a more in-depth understanding of existing knowledge and approaches to indexes and KPIs for SSC. This Supplement illustrates the vast body of resources gathered on the evaluation index systems of smart cities and KPIs for sustainable cities, among others, that served as a background for the development of related series of KPIs Recommendations and Supplements. The resources presented in this Supplement were analysed with respect to common elements, and a set of indicators focusing on ICT and its contribution to smart sustainable cites was developed.