Metamorphic rocks are rocks that have undergone a transformation from an existing rock to a new type of rock through a process called metamorphism. This process involves subjecting the original rock, known as the protolith, to high temperatures greater than 150 to 200 °C and often elevated pressure of 100 megapascals or more. The heat and pressure cause the minerals in the protolith to recrystallize and reorganize, resulting in a new type of rock with different physical and chemical properties. Some examples of metamorphic rocks include gneiss, slate, marble, schist, and quartzite.

Metamorphic rocks are formed from existing rocks that are transformed physically or chemically at elevated temperatures, without actually melting to any great degree. The importance of heating in the formation of metamorphic rock was first noted by the pioneering Scottish naturalist, James Hutton, who is often described as the father of modern geology.

Metamorphic rocks are one of the three great divisions of rock types, along with igneous rocks, which form from molten magma, and sedimentary rocks, which form from sediments eroded from existing rock or precipitated chemically from bodies of water.

Metamorphic rocks have various uses in construction and engineering. For example, slate and quartzite tiles are used in building construction, while marble is prized for building construction and as a medium for sculpture. However, schist bedrock can pose a challenge for civil engineering because of its pronounced planes of weakness.