Abstract: Emerging defence systems need to be able to exploit terrain in the form of digital geographic information. The component of defence systems that will perform this critical role is the geographic information system (GIS). The implementation of GIS poses many challenges for system developers and this paper aims to expose some of the most important issues that must be addressed. Three issues are raised as typifying the complexities of GIS. The need for compromise between speed, functionality and openness in a GIS implementation is identified. The need for, and complexities of, geographic data management are discussed and finally the need for organisational change is proposed. The paper concludes by briefly examining the reasons why some defence systems continue to be planned without GIS services despite all the arguments that this is not a sensible approach.