Recent developments in dynamo theory are discussed both from a kinematic and hydromagnetic point of view. The relative importance of the α- and ω-effects of mean field dynamo theory is examined. In the case of the Earth, recent results point to a model of αω-type in which the primary force balance is between Coriolis, pressure, Lorentz forces with motion driven by buoyancy forces. Accordingly, the magnetic field is strong of order ( ρΩ σ ) 1 2 , where ρ is fluid density, σ is the electrical conductivity and Ω is the rotation rate, and predominantly azimuthal. It is large compared with the meridional magnetic field emerging at the Earth's surface. Within the wider framework of planetary magnetic fields detailed issues include the nature of the convection leading to the α-effect, the generation of azimuthal zonal motion (thermal winds, geostrophic flow and Taylor's condition) responsible for the ω-effect, and the steady or oscillatory nature of the ensuing dynamo.