The term Geographical Information System has been applied to a wide range of computing environments that provide facilities for the storage, retrieval, display and analysis of data with a spatial component. While the diversity of such systems makes generalisations difficult, it is generally acknowledged that spatial data, in either textual or graphical form presents particular processing challenges that are well met by an Object Oriented approach. As a result, Object Oriented methodologies, including Object Oriented graphics, programming, analysis and databases have received considerable attention in the geographical information systems literature recently. This explosion of interest is not yet matched by many practical products or implemented systems, however, the first vendors are beginning to enter the marketplace. This chapter argues that the most important part of the approach will be the application of Object Oriented analysis to define suitable data models and to encourage data and method sharing across Open Systems compliant networks connecting diverse hardware platforms running both existing applications and novel software.