The Earth's magnetic field has existed for hundreds of millions of years, far longer than life has existed on Earth, and it affects our lives in many ways. We can use it to orient buildings, to navigate across unmarked territory, and to protect us from harmful radiation from space. Intended as an introductory guide for nonspecialist readers, this book describes the historical importance of the Earth's magnetic field and its role in protecting the planet from harmful high-energy radiation from the Sun. With explanations of the underlying physics of processes and references to original scientific works, the reader can explore the Earth's magnetic field and the various ways in which geomagnetism is used and measured, including the analysis of modern satellite-based investigations and the effects of solar activity on the geomagnetic field.