This chapter explores the use of the Internet of Things (IoT) in improving the sustainability performance of cities. It provides an overview of IoT-enabling technologies such as barcodes, radio frequency identification (RFID) and sensors typically used in the built environment. Existing technologies such as barcodes, RFID and sensors can serve as the foundation on which to build the IoT. The chapter examines the application of these technologies in selected areas such as energy management, environmental monitoring and waste management intended to improve the sustainability performance of cities. Selected case studies from two leading smart cities, Singapore and Barcelona, are examined in detail to establish the extent of the application of the technologies and identify the anticipated benefits. The challenges of dealing with the datasets are compounded by the way they are used, including the behaviour of users that are too often unpredictable. The chapter concludes with IoT-related applications and the challenges as well as opportunities of implementing them more widely.