The idea behind this book originated at a meeting held in Caramulo in Portugal in September 2003. The participants agreed that, though the field of natural dynamos (planetary, stellar and galactic) was rapidly evolving and attracting the interest of researchers in other branches of fluid mechanics, there was no comprehensive introductory book for researchers or graduate students entering this research area. The organisers therefore decided to take advantage of the broad-based knowledge of the invited lecturers at the conference to assemble a “multi-authored monograph”. Despite the obvious contradiction in this phrase, it does reflect the spirit in which this book was prepared. While each section of the book was written by specialists in different aspects of this subject, a concerted effort has been made to provide a unified presentation, which develops concepts in a coherent order and, where feasible, uses consistent notation.