A major problem in research in Man—Machine Interactions is to develop the methods to model the tasks of an operator supervising a complex dynamic sytem. We propose a design and evaluation methodology which specify the steps wich lead to a system which is truly centred about operator's task and the technical and ergonomics design and evaluation of the supervision system. The formula of an operator's task is based on a confrontation between two models: a model of his possible behaviour and a model of his actual behaviour. The result is a model which explains the behaviour of operator. The model is described by graph theory (SADT and synchronized Petri nets) which gives a simple and formal tool directly usable by the designer. This conceptual model is then exploited for the specification of the displays. An example of application is given in the actual context of air trafic control work station.