Résumé : Model driven engineering advocates the separation of concerns during the design time of a system, which leads to the creation of several different models, using several different syntaxes. However, to reason on the overall system, we need to compose these models. Unfortunately, composition of models is done in an ad hoc way, preventing comparison, capitalisation and reuse of the composition operators. In order to improve comprehension and allow comparison of merging and weaving operators, we use category theory to propose a unified framework to formally define merging and weaving of models. We successfully use this framework to compare them, both through the way they are transformed in the formalism, and through several properties, such as completeness or non-redundancy. Finally, we validate this framework by checking that it correctly identifies three tools as performing merging or weaving of models.