The impact of various rare-earth and related doping elements (R = Lu, Sc, Yb, Y, Sin, La) on the grain growth anisotropy and the mechanical properties of polycrystalline beta-silicon nitride ceramics has been studied. Model experiments, in which Si3N4 Particles can grow freely in an R-Si-Mg-oxynitride glass matrix, show that, with increasing ionic radius of the additive, grain anisotropy increases due to non-linear growth kinetics. Toughness and strength are affected by the rare-earth element. Samples of equivalent grain sizes and morphologies yield an increasing toughness with increasing ion size of the R3+, reflecting an increasingly intergranular crack path. These samples are also strong and flaw tolerant, but the trends of strength and toughness do not exactly match. The choice of the rare-earth is essential to tailor microstructure, interfacial strength and mechanical properties. However, somewhat different trends for properties from IIIb and lanthanide additives indicate that more than the R3(+) size (i.e., purely ionic bond strength between R3+ and its neighbours) is important. The electronic structure of the R-element is responsible for the type of dopant adsorption and the properties of the interface.