Following on from Chapter 13 by Elvidge et al. on the technical specifica­ tions of night-time sensor technology, the last three chapters of this book will explore relationships between night-time data and population counts (Sutton, this chapter), levels of GDP (Gross Domestic Product) and CO2 emissions (Doll, Chapter 15) and crime deterrence (Weeks, Chapter 16). The strength of these relationships will, by inference, determine the potential of night-time data for forecasting population growth, economic activity, energy consumption and the creation of safer cities.