Oxynitride glasses combine a high refractoriness, with Tg typically >850 degrees C, and remarkable mechanical properties in comparison with their parent oxide glasses. Their Young's modulus and fracture toughness reach 170 GPa and 1.4 MPa m(.5), respectively. Most reports show good linear relationships between glass property values and nitrogen content. There is a clear linear dependence of Young's modulus and microhardness on fractional glass compactness (atomic packing density). They also have a better resistance to surface damage induced by indentation or scratch loading. The improvements stem from the increase of the atomic network cross-linking-because of three-fold coordinated nitrogen-and of the atomic packing density, despite nitrogen being lighter than oxygen and the Si-N bond being weaker than the Si-O bond. For constant cation composition, viscosity increases by similar to 3 orders of magnitude as similar to 17 eq.% oxygen is replaced by nitrogen. For rare earth oxynitride glasses with constant N content, viscosity, Young's modulus, Tg, and other properties increase with increasing cation field strength (decreasing ionic radius). Research continues to find lighter, stiffer materials, including glasses, with superior mechanical properties. With higher elastic moduli, hardness, fracture toughness, strength, surface damage resistance, increased high temperature properties, oxynitride glasses offer advantages over their oxide counterparts.