Modeling is the shared activity for both software engineering and knowledge representation. At the same time, the objectives of these two domains are not the same and both seem necessary when dealing with modeling highly structured and complex systems. To face the complexity of the systems, we try nowadays to model and to simulate them. Thus, the proposition of this paper is to present the use of UML (for Unified Modelling Language) as a mixture of both approaches. We first describe separately the recent advances made in the software engineering as well as in the knowledge representation community. We explain that, despite the same goal to describe a domain of discourse, or at least a part of it, it results in very different, although related concepts. This difference is due to the focus taken by these communities. The first community is centered on building software systems, and the second one on describing points of view on the reality. From the analysis of the differences and similarities, we show how UML can nevertheless be used for modeling in the human and social sciences.