This paper reviews investigations of porous silicon nitrides with volume fraction porosities up to 0.5. These materials have been fabricated by partial hot-pressing, partial sintering or by inclusion of fugitive additives such as starches. Irrespective of the porosity, the samples exhibit almost the same microstructural features including grain size, grain aspect ratio, and pore size. Porosity dependences of Young’s modulus, flexural strength, and fracture resistance have been investigated and as expected, these properties decrease with increasing porosity. For partial hot-pressed silicon nitrides, because of the high aspect ratio ß-Si₃N₄ grains, decreases of flexural strength and fracture toughness have been shown to be moderate compared with the much greater decrease of Young’s modulus. Thus, the strain tolerance (fracture strength/Young’s modulus) increases with increasing porosity.  For silicon nitrides containing large pores with volume fractions of ~0 to 0.25, values of relative fracture energy in the range (1-P) = 0.85 to 1 are higher by a factor of up to 1.2 than for the fully dense reference silicon nitride. Characterisation of the silicon nitride microstructures has confirmed that the matrices are the same, not only in terms of porosity but also in terms of ß-Si₃N₄ morphology, grain dimensions and grain boundary phase. This suggests that the enhanced properties seen in these materials are associated only with the effect of the pores on the mode of cracking and are not associated with increased amounts of pullout.