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QoS of a service is a set of quality attributes that bear on the services ability to satisfy stated or implied needs in an end-to-end fashion [55]. This set of quality attributes does not characterize only the service but any entity used in the path between the service and its client. Such an entity may exist in any of the three possible service levels. Thus, different QoS attributes may be used to define the QoS of a service in the application, service, and infrastructure levels. In the literature two main research approaches can be identified for specifying QoS attributes: a) QoS models, and b) Quality Specification Formalisms (QSFs). Section 6.1. 1 analyzes which are the main QoS artifacts for services which include the QoS models and the QSFs. Several QoS models and QSFs have been proposed in the research literature and by standardization groups. Section 6.1. 2 analyzes the content of a QoS model and reviews the most representative QoS models that have been proposed. Section 6.1. 3 revises proposals, both from academia and industry, in the form of QSFs used for expressing service QoS.