Service-Oriented Architecture (SOA) provides a exible so- lution for building loosely coupled distributed applications. Complex ap- plications can be designed by de ning a business process that composes a set of independent software modules called services. In this scenario, each service can be selected and bound dynamically at run-time among a set of candidates that provide the same functionality but di ers in quality of service (QoS). However, the QoS values advertised by partner services are not always ensured at run-time. In response to the dynamic execution environment, the execution of a business process has to be adapted on- the- y in case that a global QoS violation is predicted. In this paper, we introduce a QoS-aware middleware system for dynamic and adaptive ser- vice execution. The run-time service selection is modeled as an optimiza- tion problem based on user's end-to-end QoS constraints and preferences on the service composition level. In contrast to the centralized execution engine adopted by most of traditional approaches, the execution of a service composition is decentralized in the middleware. Moreover, Pro- gram Evaluation and Review Technique (PERT) and Heartbeat Failure Detector (HB) are introduced as e ective approaches to predict global QoS violations and draw appropriate adaptation decisions.