This paper introduces a present-day understanding of the data and information fusion problem and describes some aspects of the methodology, technology and software toolkit developed by the authors for the design, implementation and deployment of a class of multi-agent information fusion-related applications. The distinctive feature of the proposed technology supported by the software toolkit is that it is distributed and agent-mediated, i.e. it assumes a distributed mode of designer activity mediated by agents that perform most of the routine engineering work and support the coordination of collaborative designer activities. The above methodology, technology and software toolkit were implemented and practically used for prototyping several applications from the data and information fusion scope.