On the centenary of the International Union of Geodesy and Geophysics, this book reviews the state-of-the-art research in geomagnetism, aeronomy and space weather. Written by eminent researchers from these fields, it summarises the advances in research over the past 100 years, and looks ahead to current and emerging studies on Earth's magnetic field. It provides a comprehensive overview of the generation of Earth's magnetic field, its history and its response to external forces. Starting at the centre of the Earth, the reader is taken on a journey from the interior core and mantle, through the upper atmosphere and magnetosphere, before reaching the Sun's atmosphere and corona. The applications of this research are also discussed, particularly the societal impact of solar activity on critical infrastructures in our increasingly technologically dependant society. This book provides a valuable resource and reference to academic researchers and students in geomagnetism and aeronomy.