Our understanding of magma chambers is limited by the lack of direct observations. However, numerical simulation provide insight on the conditions needed for the formation of chambers that are molten enough to feed volcanic eruptions. We can also explore the role of gas exsolution in the rupture of a magma chamber wall and the triggering of an eruption. Model results show that the formation of large magma chambers requires magma fluxes that are much higher than those inferred from geochronological data from granite plutons. This suggests that intrusions are only rarely associated with large magma chambers. To feed an eruption, the walls of a magma chamber must fracture. Gaz exsolution is often thought a possible cause of fracturing. However, gas output fluxes are controlled by cooling and crystallization. The models show that those fluxes are too low to induce the overpressure needed for magma chamber failure. In contrast, depending on circumstances, the buoyancy of gas at the roof of the chamber can cause fracturing and eruptions.