  Recent progress in the theory of solar and stellar dynamos is reviewed.
Particular emphasis is placed on the mean-field theory which tries to describe
the collective behavior of the magnetic field. In order to understand solar and
stellar activity, a quantitatively reliable theory is necessary. Much of the
new developments center around magnetic helicity conservation which is seen to
be important in numerical simulations. Only a dynamical, explicitly time
dependent theory of alpha-quenching is able to describe this behavior
correctly.
