Geomagnetism and the dynamo: where do we stand?

We review recent developments both in the observation of the Earth's magnetic field (from the short, human life timescale, to the long, geological timescale) and in the modelling of its origin (using the numerical or the experimental approach). We attempt a confrontation of these results, coming from very different fields, and show how, when combined, they can yield a better understanding of the Earth's core dynamics. We assume prior knowledge of dynamo theory, but not of geophysics. To cite this article: E. Dormy, J.-L. Le Mouël, C. R. Physique 9 (2008).