The gross properties of the magnetic fields of the Earth and of Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune are described. Strong rotation and a liquid conducting interior, at least in part, are indicated as necessary requirements for the existence of a significant magnetic field. The magnetic fields of planetary satellites are briefly described. The interior structure of the Earth is described, conditions in the outer liquid core being of particular relevance for dynamo theory. The time variation of the dipole moment of the Earth over the last few thousand years, as inferred from archaeomagnetic studies, is discussed, and the random reversals of the dipole moment over periods of the order of millions of years, that are inferred from palaeomagnetic studies of rock magnetism, are described. The need for a dynamo theory for the Earth to explain the persistence and time-variation of its magnetic field over past millennia is established. Conditions at the core-mantle boundary are discussed, and the possible relevance of precession of the Earth’s angular velocity vector as the source of energy for the core motions responsible for dynamo action is briefly considered.