We present a policy-centered QoS meta-model which can be used by service providers and consumers alike to express capabilities, requirements, constraints, and general management characteristics relevant for SLA establishment in service aggregations. We also provide a QoS assertion model which is generic, domain-independent and conforming to the WS-Policy syntax and semantics. Using these two models, assertions over acceptable and required values for QoS properties can be expressed across the different service layers and service roles.