In distributed environments like peer-to-peer networks and Grids, where the resources to be shared are provided by many different nodes generally spanned across multiple organizations, dynamic resource discovery is a central problem. In this paper, we propose a semantic approach for the dynamic resource discovery based on ontologies for a semantically rich description of the metadata characterizing the resources to be shared, and on semantic matching techniques for dynamically and flexibly comparing ontological resource descriptions against a target resource request.