The aim of this paper is to present how simulation can help diagnosis and decision support activities in farms. A first way rests on model-based diagnosis principles : the comparison between results of a simulated behavior and reality. The question is to help the counselor to compare the results of a farm with most relevant data. A second way focuses on the cognitive dimension of decision-making processes. Simulation of the « decision-maker's behavior » is means to juge suitability between farmer's management way and farmer's expectations.
