The phase relations in and physical properties of SiAlON ceramics prepared by the hot isostatic pressing technique or by pressureless sintering with a sintering aid are reviewed. The sintering aid used is an AIN and Al2O3mixture either pure or in combination with Y2O3and/or various rare‐earth oxides. Special attention is paid to the amount and type of phase(s) formed and to how properties such as hardness (HV10), fracture toughness (KIC), and oxidation resistance vary with the sintering aid used for different types of SiAlON materials, i.e., O'‐, almost monophasic α‐, pure β‐, and mixed (α+β)‐Si