Developing decision support systems for environmental applications is an intricate, challenging task. The increasing complexity of environmental decision problems, the growing number of subjects involved and keen competition between conflicting interests make decisions and decision support difficult. Decision support systems have been developed since the 1970s to help tackle semi-structured and unstructured decision problems. Despite their popularity, the success of DSS development is uncertain and many computerised decision-support tools have failed when dealing with complex and unstructured problems. This article describes the development methodology and progress of mDSS, a decision support system for water resource management that has been developed under the European research project MULINO. The mDSS tool is designed to integrate environmental (especially hydrological) models with multiple-criteria evaluation procedures. A number of prototypes have been developed and the final version is expected at the end of the 3-year project. The system's development is driven by the experience acquired in several case studies selected in five European countries. Although the main aim of the MULINO project and the DSS is to help with increasingly complex decisions of general water management, the concepts of sustainable river basin management introduced by the water framework directive are addressed as well.