book was designed for a three-week lecturing module on the principles  of geographic information systems, to be taught to students in all education programmes at ITC as the second module in their course. A geographic information system is a computer-based system that allows to study natural and man-made phenomena with an explicit bearing in space. To this end, the GIS allows to enter data, manipulate the data, and produce interpretable output that may teach us lessons about the phenomena. There are many uses for GIS technology, and ITC, with all its different domains of scientific applications, is the living proof of this statement. Fields we have in mind are, for instance, soil science, management of agricultural, forest and water resources, urban planning, geology, mineral exploration, cadastre and environmental monitoring. It is likely that the student reader of this textbook is already a domain expert in one of these fields; the intention of the book is to lay the foundation to also become an expert user of GIS technology.