For centuries, the compass has been used for orientation and navigation, with the north-seeking tendency of its magnetized needle responding to Earth's magnetic field. The main part of the field is complicated in shape and it changes over time. As a result, magnetic maps and charts need to be updated every few years, an on-going project that requires the collection of magnetic data from satellites, dedicated surveys, and permanent observatories located around the world. In recent years, monitoring of the Earth's magnetic field has become important for mitigating the impacts of space weather. Magnetic storms, or periods of time when the field is unusually active, are caused by the dynamic interaction of the Earth's magnetic field with the Sun. It is during magnetic storms that beautiful auroral lights can be seen at high latitudes. But magnetic storms can also adversely affect the infrastructure and activities of our modern, technology-based society. Large storms can cause the loss of radio communication, reduce the accuracy of global-positioning systems, damage satellite electronics and affect satellite operations, enhance radiation levels for astronauts and high-altitude pilots, increase pipeline corrosion, and induce voltage surges in electric power grids, causing blackouts. Consequently, the science of geomagnetism is more important today than at any point in the discipline's long and colorful history.