We propose, in this master internship report, a unified formal framework for model composition. Indeed, there are various operators of model composition de fined each in its own framework. Defining a unified framework for model composition allows to compare them, to reuse them and to capitalize knowledge. Defining a formal framework allows to consider the system that these compositions represent in its entirety. This framework is built through category theory and the notion of pushout. More precisely, a category of models is defined, and the pushout construction in this category described. Then, this pushout is used to formalize several model composition operators, including model merge, model transformation and aspect weaving.