This paper presents a quality-driven approach to select component services during the execution of a composite service. The process-driven composition of Web services is emerging as a promising approach to integrate business applications within and across organizational boundaries. In this approach, individual Web services are federated into composite Web services whose business logic is expressed as a process model. The tasks of this process model are essentially invocations to functionalities offered by the underlying component services. Usually, several component services are able to execute a given task, although with different levels of pricing and quality. The paper advocates that the selection of component services should be carried out during the execution of a composite service, rather than at design-time. This selection should consider multiple criteria (e.g., price, duration, reliability), and it should take into account global constraints and preferences set by the user (e.g., budget constraints). The paper proposes a global planning approach to optimally select component services during the execution of a composite service. Service selection is formulated as an optimization problem which can be solved using efficient linear programming methods. Experimental results show that this global planning approach outperforms approaches in which the component services are selected individually for each task in a composite service. The paper also defines a Web services quality model, which includes five quality criteria: execution price, execution duration, reputation, reliability, and availability. The paper further presents a global planning approach for service selection, which is formulated as an optimization problem and solved using linear programming. The approach is validated through experiments, which show that the global planning approach outperforms the local selection approach in terms of QoS metrics and system costs.This paper presents a quality-driven approach to select component services during the execution of a composite service. The process-driven composition of Web services is emerging as a promising approach to integrate business applications within and across organizational boundaries. In this approach, individual Web services are federated into composite Web services whose business logic is expressed as a process model. The tasks of this process model are essentially invocations to functionalities offered by the underlying component services. Usually, several component services are able to execute a given task, although with different levels of pricing and quality. The paper advocates that the selection of component services should be carried out during the execution of a composite service, rather than at design-time. This selection should consider multiple criteria (e.g., price, duration, reliability), and it should take into account global constraints and preferences set by the user (e.g., budget constraints). The paper proposes a global planning approach to optimally select component services during the execution of a composite service. Service selection is formulated as an optimization problem which can be solved using efficient linear programming methods. Experimental results show that this global planning approach outperforms approaches in which the component services are selected individually for each task in a composite service. The paper also defines a Web services quality model, which includes five quality criteria: execution price, execution duration, reputation, reliability, and availability. The paper further presents a global planning approach for service selection, which is formulated as an optimization problem and solved using linear programming. The approach is validated through experiments, which show that the global planning approach outperforms the local selection approach in terms of QoS metrics and system costs.