Software systems, and in particular, Object-Oriented sys- tems are models of the real world that manipulate representa- tions of its entities through models of its processes. The real world is not static: new laws are created, concurrents offer new functionalities, users have renewed expectation toward what a computer should offer them, memory constraints are added, etc. As a result, software systems must be continuously updated or face the risk of becoming gradually out-dated and irrelevant [34]. In the meantime, details and multiple abstraction levels result in a high level of com- plexity, and completely analyzing real software systems is impractical. For example, the Windows operating system consists of more than 60 millions lines of code (500,000 pages printed double-face, about 16 times the Encyclopedia Universalis). Maintaining such large applications is a trade- off between having to change a model that nobody can understand in details and limiting the impact of possible changes. Beyond maintenance, a good structure gives to the software systems good qualities for migration towards modern paradigms as web services or components, and the problem of architecture extraction is very close to the classical remodularization problem.