This article shows how, with the appearance of the open source movement, software licenses can serve as powerful tools in the hands of commercial software publishers. They enable publishers to take advantage of the particular characteristics of the software industry, such as the presence of strong network externalities, and thus help them successfully to apply their strategies in terms of growth and valorization. In this perspective the author proposes a typology of strategies that can be served by licences (valorization, control of rival firms, cooperation of the community of programmers, dissemination with a view to establishing a standard) as well as a methodology for interpreting them from a strategic point of view. This methodology is applied to two firms, TrollTech and Sun Microsystems.