The impregnation of a fibrous preform by a liquid metal is one of the techniques used to prepare metal matrix composites (MMC). The preparation by impregnation may however lead to defects like microporosity. The micropores present in MMC result from the superposition of two processes: the trapping of gas by capillarity during the infiltration phase and the formation of additional porosity related to solidification shrinkage. These two processes are analyzed in this paper and physical models are proposed for their description. It is then shown how the formation of microporosities can be simulated numerically using these models. Finally, prospects for a better numerical simulation of the preparation of MMC by impregnation are discussed.