Toughened or tempered glass is glass that has been processed by controlled thermal or chemical treatments. It has increased strength compared with normal glass and will usually shatter into small fragments, rather than sharp shards, when broken. Tempered glass is strong, has enhanced thermal resistance, and breaks into small cuboid fragments rather than irregular shards of glass and therefore is less likely to cause injury.

Toughened glass is used when strength, thermal resistance and safety are important considerations. The most commonly encountered tempered glass is that used for side and rear windows in automobiles, used for its characteristic of shattering into small cubes rather than sharp-edged shards. Tempered glass is also used in buildings for unframed assemblies (such as frameless doors), structurally-loaded applications, and any other application that would become dangerous in the event of a human impact.

Toughened glass is made from annealed glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it to above its annealing point of about 600 °C. The glass is then rapidly cooled with forced draughts of air while the inner portion of the glass remains free to flow for a short time.

An alternative chemical process involves forcing a surface layer of glass at least 0.1mm thick into compression by ion exchange of the sodium ions in the glass surface with the 30% larger potassium ions, by immersion of the glass into a bath of molten potassium nitrate. Chemical toughening results in increased toughness compared with thermal toughening, and can be applied to glass objects of complex shape.

The term toughened glass is generally used to describe fully tempered glass but is sometimes used to describe heat strengthened glass as both types undergo a thermal 'toughening' process.

There are two main types of heat treated glass, heat strengthened and fully tempered. Heat strengthened glass is twice as strong as annealed glass whilst fully tempered glass is typically four to six times the strength of annealed glass and withstands heating in microwave ovens. The difference is the residual stress in the edge and glass surface. Fully tempered glass in the US is generally above 65 MPa whilst Heat Strengthened glass is between 40 and 55 MPa.

It is important to note that while the strength of the glass does not change the deflection, being stronger means that it can deflect more before breaking. Annealed glass deflects less than tempered glass under the same load, all else being equal.

Toughened glass must be cut to size or pressed to shape before toughening and cannot be re-worked once toughened. Polishing the edges or drilling holes in the glass is carried out before the toughening process starts. Due to the balanced stresses in the glass, damage to the glass will eventually result in the glass shattering into thumbnail sized pieces. The glass is most susceptible to breakage due to damage to the edge of the glass where the tensile stress is the greatest, but shattering can also occur in the event of a hard impact in the middle of the glass pane or if the impact is concentrated (for example, striking the glass with a point). Using toughened glass can pose a security risk in some situations due to the tendency of the glass to shatter completely upon hard impact rather than leaving shards in the windowsill.