  Measurements of London penetration depth are a sensitive tool to study
multi-band superconductivity and it has provided several important insights to
the behavior of Fe-based superconductors. We first briefly review the
"experimentalist - friendly" self-consistent Eilenberger model that relates the
measurable superfluid density and structure of superconducting gaps. Then we
focus on the \BaFe -derived materials, for which the results are consistent
with 1) two distinct superconducting gaps; 2) development of strong in-plane
gap anisotropy with the departure from the optimal doping; 3) appearance of gap
nodes along the $c-$direction in a highly overdoped regime; 4) significant
pair-breaking, presumably due to charge doping; 5) fully gapped (exponential)
intrinsic behavior at the optimal doping if scattering is removed (probed in
the "self-doped" stoichiometric LiFeAs); 6) competition between magnetically
ordered state and superconductivity, which do coexist in the underdoped
compounds. Overall, it appears that while there are common trends in the
behavior of Fe-based superconductors, the gap structure is non-universal and is
sensitive to the doping level. It is plausible that the rich variety of
possible gap structures within the general $s_{\pm}$ framework is responsible
for the observed behavior.
