Geographical information science can be defined as that branch of information science that deals with the geographical domain, or as the set of fundamental scientific questions raised by geographical information and the technologies that collect, manipulate and communicate it. Geographical information can reveal interesting patterns that point in some cases to causal mechanisms. The use of Global Positioning Systems and online services has led to numerous rich sources of real-time data, and to the empowerment of the average citizen as a maker of maps, an empowerment that is especially valuable during emergencies when a dense network of citizens can potentially replace the services of scarce and expensive mapping experts. The concept of Digital Earth was defined more than a decade ago and has since been realized in several virtual-globe services that employ hierarchical data structures to support rapid pan and zoom. Research over the past two decades has led to the identification of a series of fundamental empirical principles that are broadly true of all geographical information, and that provide the basis for the design of geographical databases and analytical methods. Rapid deployment of geographical information technologies raises a series of social issues, including privacy, that are likely to become more threatening in the future. The paper ends with a summary of some of the core challenges and future of geographical information science.