Software rejuvenation is the concept of gracefully terminating an application and immediately restarting it at a clean internal state. In a client-server type of application where the server is intended to run perpetually for providing a service to its clients, rejuvenating the server process periodically during the most idle time of the server increases the availability of that service. In a long-running computation-intensive application, rejuvenating the application periodically and restarting it at a previous checkpoint increases the likelihood of successfully completing the application execution. We present a model f o r analyzing software rejuvenation in such continuously-running applicataons and express downtime and costs due to downtime during rejuvneation an t e r n s of the parameters in that model. Threshold conditions f o r rejuvenation to be beneficial are also derived. W e implemented a reusable module to perform software rejuvenation. That module can be embedded in any existing application on a UNIX platform with minimal efort. Experiences with software rejuvenation in a billing data collection subsystem of a telecommunications operations system and other continuouslyrunning systems and scientific applications in A T & T are described.