This chapter provides a basic overview of geographic information systems (GISs) as well as a summary of basic concepts encountered with GISs. Specifically, it touches on the various spatial data structures and models used by GISs to represent geographical information. First, general concepts related to information organization and data structure are briefly described and related to the different ways of representing real-world geographical data and information in GISs. Second, different perspectives on information organization are discussed, including different types of spatial relationships processed by GISs as well as the underlying information organization structure within GISs. Finally, the concept of data is investigated, as well as the purpose of databases with respect to GISs, including the various methods of modeling real-world data, relationships, and processes into databases.