Abstract: The paper defines new performability-based QOS (quality of service) metrics to characterize the quality of services offered over telecommunication and computer communication networks. Performability metrics used to characterize fault-tolerant computer systems are not directly applicable to services. This paper discusses the various challenges that arise in defining performability metrics for services. These metrics are defined both from a service provider perspective as well as a customer perspective. In order to model real-world service offerings, which often involve multiple vendor networks, we define service segment metrics from which overall service metrics can be obtained. Additionally, these service segment metrics can be used in negotiations between the different vendors participating in the joint service offering.