Résumé : The organisation of actual design activities, even by experts involved in routine tasks, is not appropriately characterised by the retrieval of pre-existing plans, but is opportunistic (possibly with hierarchical episodes at a local level, but not globally hierarchical). Actually executed design actions depend, at each moment t, on the evaluation of actions proposed at t-1. These proposals can be made by pre-established plans, but also by other action-proposal knowledge structures. This position is supported by results from diverse empirical design studies. A major reason why design activities are organised opportunistically is that, even if designers possess plans which they may retrieve and use, the designers very often deviate from these plans so that their activity satisfy action-management constraints, of which the most important is "cognitive economy". Two types of variables underlying this opportunism are discussed: "situational" and "processing". If design is opportunistically organised, a support system which imposes a hierarchically structured design process will probably handicap designers. Suggestions for systems offering "real" support are formulated.