Companies are now economically bound to collaborate with other enterprises, forming horizontal networks that are generally goal-based and are of a limited duration. This article specifically addresses networks of industrial small and medium enterprises (SMEs). Thanks to a five-years study, some of the main characteristics of such networks will be clarified. One upshot from the experiment was that such networks are rarely guided by formal and deliberate strategies, and that their evolutions and mutations cannot be forecast with the usual strategic planning tools. This paper presents a formal framework to analysis and design the trajectories of networks of firms, with the aim of facilitating the strategic guidance of the network. First, approaches dealing with strategic evolution are presented. Next, organizational trajectories are examined, based on the relative situation of activities and competencies between the companies inside the network. Then, a typology of SMEs networks is given and illustrated by industrial cases. Finally, a formal frame based on the typology and on the degree of achievement of the network (as expressed by a six-levels Maslow's pyramid) is presented, which allows location and guidance of SMEs networks according to their needs. As an illustration a real network trajectory is also given.