New data sources, most notably remote-sensing satellite imagery, are offering exciting opportunities to understand the urban footprint and how it is changing over time. Moreover, “big data” sources, such as cell phone signals, can shed light on people’s mobility in and around cities, which could provide useful information for city planners. Governments need to develop policies and guidelines for the accessibility and use of such new data sources, including safeguards for privacy and confidentiality. Traditional data collection systems on migration, including censuses, surveys and administrative records, should be strengthened.