The aim of epidemiology is to provide the necessary data to implement animal health programmes and to control the diseases. In too many occasions, especially in developing countries, informations are still incomplete and the decision is often difficult. Using the model developed by SIMON for firm management, the authors describe a decision making process calling for the personality of the decision makers when data are incomplete (limited rationality model). The different steps of the process (intelligence, design, choice, implementation and control) are discussed.