Meteorites constitute the most abundant source of extraterrestrial material. They formed under a wide range of redox conditions and contain many minerals not found on Earth. Their study extends the range of known petrological and geochemical processes; they serve as concrete examples of shock metamorphism of natural materials. They contain the most ancient examples of organic compounds and aqueously altered minerals that can be studied in the lab. Calcium-aluminium-rich inclusions (CAIs) yield the age of the Solar System and CI chondrites provide the cosmic abundances of most elements. Meteorites can provide information about the interactions between cosmic rays and solid materials. They likely delivered raw materials to the early Earth, possibly facilitating the origin of life. Impact-crater formation by asteroids is the main geomorphological process in the Solar System; it changed the course of biological evolution on Earth. Meteorites provide clues to the geological history of asteroids, the Moon, and Mars, and many iron meteorites provide samples of planetesimal cores. Presolar grains permit the in situ examination of materials from other stars that existed long before the Solar System.