The purpose of this chapter is to provide an overview, rationale, and motivation for understanding quality of security service (QoSS). Just as with the quality of service (QoS) mechanisms from which they are derived, QoSS mechanisms benefit both the subscriber (e.g., individual user or enterprise) and the overall distributed system. Users benefit by having reliable access to services, and the distributed systems whose resources are QoSS-managed may benefit by having more predictable resource utilization by users and more efficient resource allocation. Thus, the QoSS vision is to transform security from a performance obstacle into an adaptive, constructive network management tool. Approved for public release; distribution is unlimited.