Software errors are a major cause of outages and they are increasingly exploited in malicious attacks. Byzantine fault tolerance allows replicated systems to mask some software errors but it is expensive to deploy. This paper describes a replication technique, BASE, which uses abstraction to reduce the cost of Byzantine fault tolerance and to improve its ability to mask software errors. BASE reduces cost because it enables reuse of off-the-shelf service implementations. It improves availability because each replica can be repaired periodically using an abstract view of the state stored by correct replicas, and because each replica can run distinct or nondeterministic service implementations, which reduces the probability of common mode failures. We built an NFS service where each replica can run a different off-the-shelf file system implementation, and an object-oriented database where the replicas ran the same, nondeterministic implementation. These examples suggest that our technique can be used in practice—in both cases, the implementation required only a modest amount of new code, and our performance results indicate that the replicated services perform comparably to the implementations that they reuse.