In the context of Service Oriented Architectures (SOA), Service Level Agreements (SLA) establish an actual link between the requester and the provider of a service. Their aim is to guarantee the requester on functional (e.g., scope of the service, optional services) and non-functional (e.g., service level objectives) aspects of the required service. Non-functional issues may be related to the quality of the service itself (e.g., service level indicators) and the quality of the provisioning/delivery process (e.g., restrictions to ensure the requested quality) of the service. Aiming to introduce adaptation in SOA is a challenging task because adaptation claims for the representation and monitoring of several non-functional aspects of the services and their delivery process at run-time. In this paper we present an architectural model which defines and manages SLA to achieve adaptivity at run-time.