The different scales of plastic flow in silicon nitride were investigated either by indentation experiments and compression under hydrostatic pressure in the 20-850°C temperature range, and by stress relaxation and creep above 1350°C. [0001], 1/3<11-20> and 1/3<11-23> dislocations were evidenced by Transmission Electron Microscopy (TEM) in the low temperature range. Cross-slip events in {10-10} prismatic planes were observed at temperature as low as 20°C by Atomic Force Microscopy (AFM) on micro-hardness indents. By increasing the temperature, the deviation plane becomes {11-20} prismatic planes. The easiest slip system is by far the [0001]{10-10} system. Above 1350°C, the creep strain could be fitted by the sum of a transient component, εt=ε∞[1-exp-(t/τc)bc], where τc reflects the duration of the transient creep stage, and bc is between 0 and 1, and a stationary component, εs=εst =Aσnt, where σ is the stress and n is the stress exponent. The increase of ε∞ with temperature is interpreted on the basis of the formation of liquid intergranulary phases above 1400°C by progressive melting of some of the grains. A creep exponent of 1.8 was determined. A single value could hardly be given to the activation energy since an S-shape curve was observed in the In εs versus l/T plot, as for most glasses over large temperature ranges. The stress relaxation kinetics was found to follow the Kohlrausch-Williams-Watt expression: σ/σo=exp [-(t/τr)br], where br ranges between 0 (solid state) and I (liquid state) and τr is a characteristic relaxation time constant. As in the case of glasses, τr decreases rapidly whereas br increases from about 0.2 to 0.7 as the temperature increases from 1400 to 1650°C. But again, it is very difficult to get a single value for the activation energy from the In τr versus 1/T plot.