Introduction to Geomagnetic Fields is a textbook for advanced undergraduate and graduate students of geophysics. It explains the natural magnetic fields in and surrounding the Earth that arise from a variety of electric currents. The author clearly presents these different components of the Earth's magnetic field with a minimum of mathematical complexity. Readers are also introduced to the techniques and instrumentation for measuring geomagnetic fields, and to the range of applications for which these measurements are used. This second edition has been fully revised. It has been designed as a textbook for use with semester courses in geomagnetism and includes student exercises at the end of each chapter. Special appendices review relevant mathematical techniques and direct the reader to various journals, books, organizations, and websites where computer programs for geomagnetism may be downloaded.