The Earth's magnetic field has existed for hundreds of millions of years, far longer than life has existed on Earth, and affects our lives in many ways. We can use it to orient buildings and navigate across unmarked territory. Moreover, it protects us from harmful radiation from space. Intended as an introductory guide for non-specialist 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 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 geomagnetics are used and measured, including the analysis of modern satellite-based investigations and the effects of solar activity on the geomagnetic field.