This paper presents the activity of complex process supervision in terms of reasoning : to observe, to validate, to decide, to act, such are the human activities in control rooms. In order to build a system which helps operators in making their decisions, their different tasks are first analyzed, decomposed and shown to rise from causal reasoning. A model-based approach for causal reasoning is then emphasized; the specifications for the model are detailed. An implementation based on a signed directed graph representing causality, and on simple transfer functions describing the propagation of information through the graph, is finally proposed. The capacities of this modelling method to simulate, to explain and to give action advice are demonstrated on an industrial example.