The previous chapter has discussed the rise of magma throughout the crust. This fourth chapter focuses on the arrest, emplacement and accumulation of magma. These are indeed primary and widespread processes at volcanoes, as most of the rising magma remains stalled in the crust, with only a fraction being erupted (approximately one tenth; e.g., Shaw 1985; White et al. 2006). Moreover, the accumulated magma may develop magma chambers, whose dynamics can be detected to define the state of active volcanoes, also providing a warning for forecasting any impending eruption.