Sintering models are discussed and used to analyze¯ow instabilities that may arise during preliminary compaction of powders. These instabilities can be at the origin of heterogeneities in the densi®cation. The material is modeled as a viscoplastic thermal sensitive porous material. The modeling includes the limit case of a linear viscous material. The eects of sintering conditions (temperature and pressure in the case of pressure sintering) and the eects of material characteristics such as porosity, heat capacity, theoretical density, surface tension, particle size and creep parameters on stability of sintering are investigated. The heat release associated with the plastic¯ow is shown to sometimes have an important role. Stability criteria are derived and applied to the analysis of sintering and hot isostatic pressing, using various sintering models. These stability criteria can be used to optimize the densi®cation process; one can control, for example, temperature so as to avoid any instability. Stability maps enabling an optimization of temperature±pressure regime in hot isostatic pressing are built for sample metal (nickel) powder. #