In this paper, a case study in regional water resources planning is presented in which the process of design and use of computer-based models and man-computer interaction is considered, especially with respect to the roles of designers and potential users. Key decisions in the design process are recognized and analyzed. It is concluded that integration of normative models in decision processes with conflicts shows limits that partly may be overcome by emphasis on the planning stage of the design process, and on development of planning and decision models that allow flexibility. Interactive procedures should be developed that distribute flexibility over decision levels and also deal with the problems of asymmetrical distributed information. It is concluded further that there is a need for a more systematical investigation of design processes and decision behavior related to characteristics of decision problems and environments.