Nigel Weiss discusses dynamos in settings as diverse as galaxies and planets in this his 2000 Presidential Address to the Ordinary (A&G) Meeting of the Royal Astronomical Society. Abstract Global magnetic fields in the Earth and other major planets, in the Sun and other active stars, and also in spiral galaxies like the Milky Way, are apparently maintained by hydromagnetic dynamos. This lecture will contrast the various dynamo models that have been put forward. Powerful supercomputers have now made it possible to simulate the geodynamo in considerable detail. In the case of the Sun, we have yet to explain its internal rotation and models still have to rely on mean field dynamo theory. There are many features that can be explained but extrapolation to other stars remains uncertain. For galaxies, even the need for a dynamo is still controversial.