In this paper, we present the self-describing schemes for interoperable image/video content descriptions, which are being developed as part of our proposal to the MPEG-7 standard. MPEG-7 aims to standardize content descriptions for multimedia data. The objective of this standard is to facilitate content-focused applications like multimedia searching, filtering, browsing, and summarization. To ensure maximum interoperability and flexibility, our descriptions are defined using the eXtensible Markup Language (XML), developed by the World Wide Web Consortium. We demonstrate the feasibility and efficiency of our self-describing schemes in our MPEG-7 testbed. First, we show how our scheme can accommodate image and video descriptions that are generated by a wide variety of systems. Then, we present two systems being developed that are enabled and enhanced by the proposed approach for multimedia content descriptions. The first system is an intelligent search engine with an associated expressive query interface. The second system is a new version of MetaSEEk, a metasearch system for mediation among multiple search engines for audio-visual information.