A material placed inside a prepared cavity prior to the placement of a restoration. Linings are used for protective, therapeutic, or structural reasons. Deep cavities may benefit from a lining material providing thermal protection, particularly for metallic restorations, although an additional sublining may be necessary. A therapeutic lining material may be placed to stimulate the odontoblasts to lay down reparative dentine, to encourage remineralization, or to discourage bacterial activity. Structural linings reduce the volume of restorative material required and restore the internal form of the cavity; this can be advantageous for cast metal restorations by removing undercut areas and reducing the bulk of the restorative material. Lining materials include calcium hydroxide, resin modified glass ionomer cements, zinc-oxide eugenol, zinc phosphate, and glass ionomer.