The Earth's magnetic field shields our habitat against potentially harmful charged particles from outer space. Moreover, it is an important source of information about Earth's deep interior and geological history, both inaccessible to direct observations. Geomagnetism, the study of all aspects of the Earth's magneticfield, formed as a field of natural sciences in the early nineteenth century, largely under the influence of two prominent polymaths, Alexander von Humboldt and Carl Friedrich Gauss. On the occasion of the AGU 2019 centennial and Alexander von Humboldt's 250th anniversary, we link Humboldt's activities in geomagnetism with modern research in this domain and its challenges. Based on magnetic field observations during his scientific journeys, Alexander von Humboldt determined the increase of magnetic intensity with distance from the equator. He initiated coordinated observations across the globe and thus laid the foundation for international data exchange and collaboration. Observation is a prerequisite to a better understanding of the Earth's magnetic field with its various sources from the Earth's core, lithosphere, and electrical current systems outside the Earth, with a multitude of applications from navigation, exploration, natural hazard assessment, and study of Earth's evolution. A full understanding of the physical processes underlying the various geomagnetic field sources is mandatory to tackle the challenge of forecasting the future geomagnetic field evolution and to take advantage of the information that the geomagnetic field can provide about the Earth's history, geological and tectonic conditions and processes, and the near‐Earth's space environment. Plain Language Summary Earth's magnetic field shields our planet against highly energetic particles from the Sun and outer space, which threaten modern technology. The main part of the magnetic field originates from dynamical processes in the Earth's outer, fluid core, and changes slowly. Additional contributions are due to magnetized rocks in the uppermost parts of the Earth. Moreover, several quickly varying electrical current systems flowing at distances from approximately 60 km to several Earth's radii around our planet contribute to geomagnetic field observations. Alexander von Humboldt played an important role in establishing the science of geomagnetism. He described the systematic change of magnetic field strength with distance from the equator, and he initiated synchronized magnetic field observations worldwide. Since that time, significant progress has been made in understanding the various geomagnetic field sources, and we have recognized applications ranging from understanding the Earth's geological history, exploration for natural resources, natural hazard assessment, navigation, and protection against adverse space weather influences. Main challenges in geomagnetic field research include a clear separation of signals from the individual field contributions and a full understanding of their physical sources, a more complete understanding of the long‐term history of the geomagnetic field, and forecasting future field changes. Key Points Alexander von Humboldt's role as one of the founders of geomagnetism is presented Current understanding of the various aspects of geomagnetism and paleomagnetism is summarized Grand challenges in geomagnetism and its societal relevance are outlined