  We investigate departures from local thermodynamic equilibrium (NLTE) in the
line formation of neutral and singly ionised iron lines and their impact on
spectroscopic stellar parameters. The calculations are performed for an
extensive grid of 1D MARCS models of metal-rich and metal-poor late-type dwarfs
and giants. We find that iron abundances derived from FeI lines are
increasingly underestimated in hotter, lower surface-gravity, and more
metal-poor stars, in a simple and well-defined pattern, while LTE is usually a
realistic approximation for FeII lines. For the vast majority of dwarfs and
giants, the perturbed ionisation balance of FeI and FeII is the main relevant
NLTE effect to consider in the determination of spectroscopic parameters, while
for extremely metal-poor stars and hot giant stars significant impact is seen
also on the excitation balance and on the microturbulence determination from
FeI lines.
