Dynamo theory, the study of the generation and maintenance of magnetic fields by fluid motions, is important in many areas of physics, ranging from stellar and galactic dynamics, through solar physics and geomagnetism, to reactor physics. This volume contains the lectures given by leading specialists, for an intensive course held at the Newton Institute, as part of a NATO Advanced Study Institute. The course was intended for beginning graduate students, so starts with background material, before leading on to describe areas closer to current research. Topics covered include planetary and solar dynamos, fast dynamos and the use of symmetry principles to derive evolution equations. Detailed bibliographies are provided. As the only modern introduction to the subject, this will be welcome reading for students in planetary and solar physics, plasma physics and astrophysics.