The big data era is the inevitable consequence of datafication: our ability to transform each event and every interaction in the world into digital data, and our concomitant desire to analyze and extract value from this data. Big data comes with a lot of promise, enabling us to make valuable, data-driven decisions to alter all aspects of society. Big data is being generated and used today in a variety of domains, including data-driven science, telecommunications, social media, large-scale e-commerce, medical records and e-health, and so on. Since the value of data explodes when it can be linked and fused with other data, addressing the big data integration (BDI) challenge is critical to realizing the promise of big data in these and other domains. As one prominent example, recent efforts in mining the web and extracting entities, rela- tionships, and ontologies to build general purpose knowledge bases such as Freebase [Bollacker et al. 2008], the Google knowledge graph [Dong et al. 2014a], ProBase [Wu et al. 2012], and Yago [Weikum and Theobald 2010] show promise of using integrated big data to improve applica- tions such as web search and web-scale data analysis. As a second important example, the flood of geo-referenced data available in recent years, such as geo-tagged web objects (e.g., photos, videos, tweets), online check-ins (e.g., Foursquare), WiFi logs, GPS traces of vehicles (e.g., taxi cabs), and roadside sensor networks has given momentum for using such integrated big data to characterize large-scale human mobility [Becker et al. 2013], and influence areas like public health, traffic engineering, and urban planning. In this chapter, we first describe the problem of data integration and the components of traditional data integration in Section 1.1. We then discuss the specific challenges that arise in BDI in Section 1.2, where we first identify the dimensions along which BDI differs from traditional data integration, then present a number of recent case studies that empirically study the nature of data sources in BDI. BDI also offers opportunities that do not exist in traditional data integration, and we highlight some of these opportunities in Section 1.3. Finally, we present an outline of the rest of the book in Section 1.4.