Developing system models for agricultural development is time consuming. Efficient use of resources requires that one anticipate the various activities of a modeling project. The purpose of this paper is to provide an aid for the planning and execution of modeling projects by presenting how 20 different modeling projects in France were carried out. We can distinguish two types of project. The first group aims at the development of decision support software for farmers or extension personnel for application to their specific situation. The second group aims at developing software which will be used by researchers and agricultural engineers to evaluate agricultural production systems over a range of typical situations. To be efficient, it is important to clearly define the type of project from the start.