Geographical Information Systems (GIS) are computer systems for storing, displaying and analysing spatial data. The past twenty years have seen a rapid growth in their use in government, commerce and academia, and they can be used for managing a network of utilities, from handling census data through to planning the location of a new supermarket. But how do they work? Stephen Wise has been a regular contributor to GeoEurope and his 'Back to Basics' articles have provided a clear and simple introduction to the inner workings of GIS for a non-specialist audience. He now presents the original articles with new material and provides a new coverage of both major types of GIS - vector and raster systems. Undergraduates and professionals who wish to improve their knowledge of GIS should get a better understanding of how GIS operate in the way that they do, such as how spatial data is stored on a computer, how the different methods affect the capabilities of the GIS, how basic operations performed and how the choice of algorithm affects the speed of the system.