Casing may refer to an enclosing shell, tube, or surrounding material. It may also refer to:

Cartridge (firearms), shell enclosing the explosive propellant in ammunition
Casing (borehole), metal tube used during the drilling of a well
Casing (molding), decorative molding surrounding door or window openings
Casing (sausage), thin covering holding the food contents of sausage
Casing (submarine), platform attached to the upper side of a submersible vehicle
Computer case, the enclosure that contains most of the components of a computer
Letter case, the distinction between upper and lowercase letters in typography
Surreptitious reconnaissance, especially to aid a robbery