Toughened or tempered glass is glass that has been processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempered glass is made by processes which create balanced internal stresses which give the glass strength. It will usually shatter into small fragments instead of sharp shards when broken, making it less likely to cause severe injury and deep lacerations. As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including passenger vehicle windows, glass doors and tables, as a component of bulletproof glass, for diving masks, and various types of plates and cookware.

Toughened glass is physically and thermally stronger than regular glass. The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. For glass to be considered toughened, this compressive stress on the surface of the glass should be a minimum of 69 MPa. For it to be considered safety glass, the surface compressive stress should exceed 100 MPa. The greater the surface stress, the smaller the glass particles will be when broken. It is this compressive stress that gives the toughened glass increased strength. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin. However, the toughened glass surface is not as hard as annealed glass and is therefore somewhat more susceptible to scratching. To prevent this, toughened glass manufacturers apply various coatings and/or laminates to the surface of the glass.

Any cutting or grinding must be done prior to tempering. Cutting, grinding, sharp impacts and sometimes even scratches after tempering will cause the glass to fracture.

The strain pattern resulting from tempering can be observed with polarized light or by using a pair of polarizing sun glasses.