Topics involved in studies of the Earth's magnetic field and its secular variation range from the intricate observations of geomagnetism, to worldwide studies of archeomagnetism and paleomagnetism, through to the complex mathematics of dynamo theory. Traditionally these different aspects of geomagnetism have in the main been studied and presented in isolation from each other. This text draws together these lines of inquiry into an integrated framework to highlight the interrelationships and thus to provide a more comprehensive understanding of the geomagnetic field. The text is organizd so that paleomagnetists and dynamo theoreticians may both benefit from the results and arguments presented by the other. A particular example is the presentation of paleomagnetic results to illuminate the observational constraints on geodynamo theory. Conversely, dynamo theory is explained in such a way that paleomagnetists may utilize it to present their data more effectively. Other important features of the book include a discussion of planetary magnetic fields and their implications for dynamo theory and the most recent set of magnetic charts.