Tailoring the microstructure and the composition of silicon nitride ceramics can have profound effects on their properties. Here it is shown that the grain growth behavior, in particular its anisotropy, is a function of the specific additives, which allow one to tune the microstructure from one consisting of more equiaxed grains to one with very elongated grains. Recent studies are discussed that provide an understanding of the atomic level processes by which these additives influence grain shapes. Next the microstructural (and compositional) parameters are discussed that can be used to modify the thermal conductivity, as well as fracture toughness of silicon nitride ceramics. As a result of the open <0001> channels in $eta-Si_3N_4$, the c-axis conductivity can be exceptionally high. Thus, the formation of elongated c-axis grains, particularly when aligned can result in conductivity values approaching those of AlN ceramics. In addition, the controlled formation of elongated grains can also be used to significantly enhance the fracture toughness. At the same time, both properties are shown to be affected by the composition of the densification additives. Utilizing such understanding, one will be able to tailor the ceramics to achieve the properties desired for specific applications.