Casting is a manufacturing process by which a molten material such as metal or plastic is introduced into a mold, allowed to solidify within the mold, and then ejected or broken out to make a fabricated part. Casting is used for making parts of complex shape that would be difficult or uneconomical to make by other methods (such as cutting from solid material).

It has been used for over 5,000 years to create art and tools. The earliest casting was a copper frog dating back to 3200 B.C.E., cast in Mesopotamia. Today, it is used all over the world for everything from making huge tools to small artworks such as jewelry. In the future, casting may be used for even things not metal. One purpose that is being tested now is using a mold to create an artificial heart valve. Instead of using plastic, though, they are using live human cells in a mixture of resin to make a living valve.

Casting may be used to form hot, liquid metals or meltable plastics (called thermoplastics), or various materials that cold set after mixing of components such as certain plastic resins (e.g. epoxy), water setting materials such as concrete or plaster, and materials that become liquid or paste when moist such as clay, which when dry enough to be rigid is removed from the mold, further dried, and fired in a kiln.

Substitution is always a factor in deciding whether other techniques should be used instead of casting. Alternatives include parts that can be stamped out on a punch press or deep-drawn, forged, items that can be manufactured by extrusion or by cold-bending, and parts that can be made from highly active metals.