Data from 'human sensors' is increasingly easy to collect. Yet how may systems be designed that put it to use? This chapter discusses this question in three steps. First, we describe how the increasing ubiquity of digital systems is facilitating the creation of streams of human data. We characterise these data sources according to their purpose, obtrusiveness, structure, and hierarchy. Then, we address the kinds of systems that are already reaping the benefits of these data sources; they are

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... adly categorised as recommendation, retrieval, and behaviour-mediating systems. Finally, we describe a case study of potential systems that may be built to support urban travellers by leveraging the data that travellers themselves create while navigating their city. The chapter concludes with three open research challenges, related to understanding the context of data creation, the systems that are designed to use this data, and how to best architect a bridge between the two.