Web service based applications are expected to live in dynamically evolving settings. At run-time, services may undergo changes that could modify their expected behavior. Because of such intrinsic dynamic nature, applications should be designed by adhering to the principles of designby-contract. Run-time monitoring is needed to check that the contract between service providers and service users is fulfilled while the collaboration is in place. We describe a language to specify the expected

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... ional and nonfunctional requirements that a service provider should fulfill. The language (Timed WSCoL) is a temporal extension of a previous proposal (WSCoL). We also illustrate the architecture of a run-time analyzer that checks Timed WSCoL properties. Should such properties be disproved during execution, appropriate recovery and reconfiguration actions may be put in place. * Part of this work has been supported by the IST EU project "PLAS-TIC", the IST EU project "SeCSE", the italian FIRB project "ART DECO" and the italian FAR project "Discorso".