Smart cities are a fast-growing species, and a fascinating field for new experiments in a number of critical areas, ranging from urban planning, sustainable energy, and transport strategies to social integration and talent attraction, to name a few. As leaders and citizens around the world continue to assess, design, implement and improve on ways to create better cities, they often find themselves confronted with a multitude of decisions and a wide range of partial solutions to specific problems such as traffic congestion, waste management and crime. Unfortunately, they have precious few tools to enable them to define the strategies required and take advantage of the experience of other smart cities around the world. In such a context, metrics can play a significant and constructive role: by quantifying efforts and results, they increase the ability of decision-makers to identify where their priorities should lie as well as the relative merits of various approaches.