The fracture energy of Si3N4 made by hot pressing, reaction sintering, and chemical vapour deposition (CVD) was studied. Extrapolation of fracture energies to zero additive or porosity levels, as well as analysis of CVD Si3N4 all indicate an intrinsic fracture energy of 20–30J m−2. Higher fracture energies in dense bodies with increasing additive content, or in some more porous bodies (relative to expected porosity dependence) are associated with crack branching. In dense bodies such branching may arise due to micro-cracking from combined effects of crack tip stresses and mismatch stresses due to differences in properties, especially thermal expansion, between Si3N4 and the additive or its reaction products. In porous bodies such branching appears to be due to spatial distribution of pores.