Rock magnetism is the study of induced and remanent magnetization of ferrimagnetic mineral grains in rocks, sediments, soils, and organisms. Its applications include environmental magnetism, magnetic anisotropy, sources of continental and oceanic magnetic anomalies, records of geomagnetic field variations and polarity reversals, and the paleomagnetic record of plate motions and the Wilson cycle. This paper reviews the beginnings of rock magnetism and then traces the development of six particularly interesting areas: pseudo-single-domain behavior; magnetic domains and micromagnetic structures; diagnostic tests of the type and stability of remanent magnetization; magnetic microanalysis; thermoviscous remagnetization; and chemical remanent magnetization. Other areas, including sediment and soil magnetism, are covered in a companion paper by Verosub and Roberts on environmental magnetism. ¿ American Geophysical Union 1995