Acoustic emission measurements aim at understanding and distinguishing all the phenomena that occur during powder compaction of pharmaceutical products. Those phenomena are granular rearrangement, fragmentation and visco-plastic deformation of grains or granules. Other considerations such as specific area and porosity evolutions versus compaction pressure provide valuable data on domains where whether fragmentation or visco-plastic deformation could be predominant. Relationships between acoustic emissions and such phenomena occurring during comp action of pharmaceutical powders are proposed. For instance, compaction of brittle products like aspirin or saccharose produces strong acoustic emission whereas compaction of starch exhibiting a high visco-plastic behaviour produces a low acoustic emission.