The evolution of Geographic Information Systems (GIS) has witnessed an important step towards achieving interoperability through the communication, exchange, cooperation and sharing of resources between systems. The main developments in interoperability include the use of standards for transfer and exchange of geographical information, the integration of different data types up to the development of comprehensive interoperability models. This article aims to review the main advances in interoperability of GIS and provide a basis to identify the future research issues in this field. It first considers the various points of view from which interoperability can be defined and then examines the concept of interoperability itself. An overview is provided on how interoperability can be modelled by discussing the main aspects that directly have an effect on how interoperability levels have been created according to different contexts and purposes of use. It also points out the significant advances in terms of measures that can determine the ability of a GIS to interoperate with other systems, predict the resources needed for successful interoperation, and discover techniques useful to achieving interoperability. We conclude with some suggestions where the research will go next.