This paper presents the design approach and architecture of a Decision Support System (DSS) for the Hellenic Centre for Emergency Health Care (EKAB, http://www.ekab.gr). The DSS supports the cooperation and decision-making processes at the EKAB call centre concerning the effective activation and allocation of appropriate resources mainly: ambulances, healthcare personnel and other public emergency resources like fire fighting and police. The DSS is a geographic web-based mash-up that builds on top of existing information systems and databases, and collects, aggregates, records, and presents various types of dynamic information about medical incidents in real time, promoting evidence-based medicine. The system provides a number of user interfaces (web-based and mobile) for call centre operators, radio centre operators, ambulance personnel, and administration. A contextual approach was necessary for the design of the system based on various related methods. Field observation of the current ways of work at EKAB sites, design, and development of data resources and user interfaces were conducted.