Magnetic fields are observed to exist wherever there is matter in the visible universe; they exist on planetary, stellar and galactic length-scales, indeed wherever there is a sufficiently large mass of rotating conducting fluid. Dynamo theory is that branch of fluid mechanics that seeks to explain both the origin of these magnetic fields, and the manner of their variation in space and time. The subject has exerted a profound challenge, and great advances have been made over the last few decades. Nevertheless, acute problems remain in relation to both planetary and stellar magnetism. The NATO Advanced Study Institute on Stellar and Planetary Dynamos, and the six-month Dynamo Theory Programme of the Isaac Newton Institute of which it formed part, set out both to review the present state of knowledge in this broad field, and to define the critical problems that now demand attention.

The problem of the origin of the Earth's magnetic field has challenged the imagination of great scientists of past centuries. Edmund Halley showed extraordinary prescience three hundred years ago when, in considering the possible causes of the secular variation of the geomagnetic field, he wrote:

This view of the inner structure of the Earth was not confirmed till Jeffreys' discovery in 1926 of the liquid outer core and Bullen's discovery in 1946 of the solid inner core.