Abstract. Huge amounts of digital visual content are currently available, thus placing a demand for advanced multimedia search engines. The contribution of this paper is the presentation of a search engine that is capable of retrieving images based on their keyword annotation with the help of an ontology, or based on the image content to find similar images, or on both these strategies. To this end, the search engine is composed of two different subsystems, a low-level image feature analysis and retrieval system and a high-level ontology-based metadata structure. The novel feature is that the two subsystems can co-operate during the evaluation of a single query in a hybrid fashion. The system has been evaluated and experimental results on real cultural heritage collections are presented.