  The theory of mean field electrodynamics, now celebrating its fiftieth
birthday, has had a profound influence on our modelling of cosmical dynamos,
greatly enhancing our understanding of how such dynamos may operate. Here I
discuss some of its undoubted triumphs, but also some of the problems that can
arise in a mean field approach to dynamos in fluids (or plasmas) that are both
highly turbulent and also extremely good electrical conductors, as found in all
astrophysical settings.
