Your GIS server experiences a hardware fault or a power failure, but your emergency 911 system is dependent on continued operation of your computer system. Your customers report a power outage in their neighborhood, just as you experience a network failure, or your server crashes from a software problem. Will your work crew be able to locate the source of the power outage? One of the disks storing your GIS database crashes. Can your users continue to work productively and without interruption? GIS user applications are "available" only if they allow users to access the GIS server application(s) and the GIS data files. High availability environments are designed for computing installations that require critical systems to be automatically and seamlessly restarted in the event of a hardware failure. They can ensure that data remains accessible and that applications are kept running, even during a prolonged hardware failure. This paper will investigate the nature and architecture of a high-availability GIS: the use of standard hardware and software components to provide automatic failover and continuous operation in the event of system failure. It will demonstrate how interruptions to applications (running on the server or on workstations and PCs networked to the server) can be minimized and how file systems can remain continuously available.