Recent research concerning the control of complex systems stresses the systemic character of the work of the controlling system, including the number of people and artefacts as well as the environment. This study adds to the growing body of knowledge by focusing on the internal working of such a system. Our vantage point is the theoretical framework of distributed cognition. Through a field study of an emergency co-ordination centre we try to demonstrate how the team's cognitive tasks, to assess an event and to dispatch adequate resources, are achieved by mutual awareness, joint situation assessment, and the co-ordinated use of the technology and the physical arrangement of the co-ordination room.