  For more than twenty years, strong Fe II emission lines have been observed in
Active Galactic Nuclei and in particular in Narrow Line Seyfert 1 galaxies. A
quick overview of the observations and of the models proposed to interpret the
Fe II spectrum is given. The influence of atomic data and of physical
parameters are discussed, and it is shown that the strengths of the Fe II lines
cannot be explained in the framework of photoionization models. A non-radiative
heating, for instance due to shocks, with an overabundance of iron, can help to
solve the problem. A comparison with other objects emitting intense Fe II lines
favors also the presence of strong outflows and shocks. We suggest some issues
in the context of AGN evolution.
