In this presentation, we will summarize our understanding of Massive-Material-Injection (MMI) -triggered disruptions, which has significantly progressed in recent years thanks to Shattered Pellet Injection experiments in several tokamaks and an intense modelling effort, for example with the 3D non-linear MHD code JOREK and the 1.5D transport code INDEX. We will also discuss still unclear points and how to address them. After reviewing the temporal and spatial scales involved, we will see how an MMI can generate a cold front and a cold helical region. We will explain why both of these probably play a key role in triggering the Thermal Quench (TQ) via destabilizing the m=2, n=1 tearing mode. We will see, however, that such regions are not always generated in simulations and discuss possible reasons and implications. We will then focus on the dynamics of the MHD relaxation associated to the TQ, with special attention to field line stochasticity and implications for Runaway Electron (RE) generation. We will explain why reproducing the experimental Ip spike is probably important for the credibility of 3D non-linear MHD simulations and their relevance for RE generation studies. Finally, we will discuss the question of the global radiative collapse at the end of the TQ, which is also critical for RE generation.