FUSE is a lightweight failure notification service for building distributed systems. Distributed systems built with FUSE are guaranteed that failure notifications never fail. Whenever a failure notification is triggered, all live members of the FUSE group will hear a notifica- tion within a bounded period of time, irrespective of node or communication failures. In contrast to previous work on failure detection, the responsibility for deciding that a failure has occurred is shared between the FUSE service and the distributed application. This allows applications to implement their own definitions of failure. Our expe- rience building a scalable distributed event delivery sys- tem on an overlay network has convinced us of the use- fulness of this service. Our results demonstrate that the network costs of each FUSE group can be small; in par- ticular, our overlay network implementation requires no additional liveness-verifying ping traffic beyond that al- ready needed to maintain the overlay, making the steady state network load independent of the number of active FUSE groups.