The question ‘what use is mineral magnetism?’ is addressed in this chapter. Magnetic rock-forming minerals can record magnetic information on timescales that exceed the age of the Earth, which enables paleomagnetism to underpin the global plate tectonic paradigm and provide the geomagnetic polarity timescale that is used to calibrate geological time. Paleomagnetic analysis also enables understanding of terrestrial and extraterrestrial magnetic fields and planetary processes that generate these fields and their variations through time. Environmental magnetism exploits the sensitivity of magnetic minerals to environmental processes at Earth’s surface, which facilitates understanding of the climatic, tectonic, or other driving forces of environmental change. The magnetic properties of nanoparticles are also exploited by organisms, as studied in biomagnetism, and are manipulated by humans in industrial, technological, and medical applications, which makes mineral magnetism useful in an exceptional range of fields. This book serves workers in these fields by providing an introduction to mineral magnetism and in-depth treatments of the magnetic properties of terrestrial magnetic minerals.