Workflow management systems usually interpret a workflow definition rigidly, allowing no deviations during execution. However, there are real life situations where users should be allowed to deviate from the prescribed static workflow definition for various reasons, including lack of information about parameter values and unavailability of the required resources. To flexibilize workflow execution, this thesis proposes an exception handling mechanism that allows the execution to proceed when otherwise it would have been stopped. The proposal is introduced as a set of extensions to OWL-S, the language adopted to define workflows, and is based on process and resource ontologies that capture the semantic information needed for the flexibilization mechanism. The thesis also defines an operational semantics for a fragment of OWL-S and the proposed extensions, as well as a distributed architecture for a workflow management system. In particular, the semantics define a form of transactional behavior for the execution of a workflow instance, in the sense that it guarantees that either all actions executed by the instance terminate correctly or they are all abandoned.