The main magnetic field of the Earth is a complex physical phenomenon. This book presents in detail the foundations of geomagnetism, developed from first principles. The book starts with an overview of the phenomena of interest in geomagnetism, and then goes on to deal with the phenomena in detail, building the necessary techniques in a thorough and consistent manner. Students and researchers will find this book to be an invaluable resource in the appreciation of the mathematical and physical foundations of geomagnetism.