We, NICT, recently started a new project for research and development of 'knowledge cluster systems' for knowledge sharing, analysis, and delivery among remote knowledge sites. We introduce several key concepts of the knowledge cluster systems. The 'Three-site model' for knowledge system architecture defines three roles of remote sites: knowledge capture, knowledge transfer, and knowledge provision, with respect to the lifecycle of knowledge communication. The 'global knowledge grid' is as an infrastructure that is suitable for implementing knowledge cluster systems on the basis of the three-site model. The knowledge cluster systems build an evolving network of community knowledge by connecting heterogeneous knowledge bases. The 'global risk management system' is being developed as an application of the knowledge cluster systems.