  Some recent results and open issues in magnetic dynamo theory are addressed.
The distinction between small-scale and mean-field dynamo (MFD) action in
forced turbulent flows is emphasized. Though useful, the MFD has been
controversial. This is partly due to concerns about its need for helically
forced turbulence, but mainly because simple "textbook" treatments are
kinematic and linear. The non-linear backreaction of the growing magnetic field
has been suspected to prematurely quench MFD action. To resolve the
controversy, we must first understand those non-linear MFDs which can be
numerically simulated. Recently, there has been progress on this front. For
simple MFDs in closed systems, dynamical quenching models that incorporate a
transfer of magnetic helicity between small and large scales agree reasonably
well with fully 3-D numerical periodic box simulations. Unresolved issues such
as the quenching of turbulent diffusion and the additional physics needed to
apply these results to real systems are also discussed herein. The following
summarizes conceptual progress in describing mean-field magnetic energy growth
in the simplest MFDs:
  For a closed turbulent flow, the non-linear mean-field dynamo, is first fast
and kinematic, then slow and dynamic, and magnetic helicity transfer makes it
so.
