By measuring acoustic emission (A.E.), it is possible to understand and to identify many of the phenomena that occur during powder compaction of pharmaceutical products. Those phenomena are granular rearrangement, fragmentation, visco-plastic deformation of grains or granules. Other data, such as specific area and porosity evolution vs. compaction pressure, provide valuable information on domains where either fragmentation or visco-plastic deformation is important. As examples, brittle products like aspirin or saccharose produce a strong acoustic emission, whereas starch exhibiting a high visco-plastic behaviour produces a low acoustic emission.