Fault-tolerant behavior is an important non-functional requirement for systems that involve high criticality. We present a framework which allows the analysis of faulttolerant behavior to be undertaken. This framework is based on the notion of state machines and tolerance relations. Results concerning fault-tolerant behavior of finite-state machines are presented and an illustrative example is discussed. Various kinds of fault-tolerant behavior (masking, fail-stop, t-fail-stop, degradable, etc.) are modeled.