Increasingly, cities are harnessing the power of technology to resolve environmental challenges. However, there remains a lack of clarity about how best to use existing and emerging technologies to achieve measureable improvements in urban sustainability. This report examines how seven cities on four continents – Barcelona, Boston, Bristol, Guangzhou, Hannover, Melbourne and Rotterdam – are addressing sustainability and the role of technology in defining ‘green’ priorities and accelerating progress. The challenges these cities are tackling include energy efficiency, road traffic congestion and overheating due to an urban ‘heat island’ effect. Their responses offer ideas that other cities may be able to adapt to tackle their own particular environmental challenges.