Three mechanisms for the creep of silicon nitride are discussed with regard to the microstructural variables that control creep. During the initial stages of creep, the amorphous silicate layer that coats all grains of silicon nitride is displaced from grain boundaries where the pressure is high to boundaries where the pressure is low. Once the amorphous layer has been entirely displaced from the grain boundaries, other mechanism of deformation take over: solution precipitation in compression; cavity formation in tension. Models of deformation for these three mechanisms suggest that the most important variable for improving creep resistance is the apparent viscosity of the intergranular phase, followed by the volume fraction of intergranular phase. In tension, grain size does not play the same important role in controlling creep as it does in compression.