This work proposes a knowledge-based approach for multimedia adaptation using Semantic Web Services technology as a tool for declaratively describing the semantics of adaptation services and constructing multi-step adaptation sequences in an open and extensible multimedia adaptation framework. It shows how the semantics of adaptation operations can be captured, how the problem of finding adequate adaptation sequences can be viewed as an Artificial Intelligence planning problem, how the existing MPEG standards are technically integrated into the service descriptions and how they serve as the shared ontology of the domain. This work on the one hand introduces declarative, knowledge-based technology into the involved multimedia communities and on the other hand broadens the application scope of Semantic Web Service technology in the area of general semantic service descriptions.