Recent numerical simulations of convection driven dynamos in rotating spherical fluid shells have revealed many features resembling those of geomagnetism such as the predominance of the dipolar polarity aligned with the axis of rotation, reversals and torsional oscillations. Since the parameters used in the computations are still far removed from those characterising the Earth's core reliable quantitative comparisons between theory and observations are not yet possible. This may change in the near future as new observations from spacecrafts and from paleomagnetic sites become available. The Prandtl number dependence of the geodynamo will be discussed and the possibility of "invisible" oscillations in the Earth's core will be pointed out.