  High energy electron irradiation is an efficient way to create
vacancy-interstitial Frenkel pairs in crystal lattice, thereby inducing
controlled non-magnetic point - like scattering centers. In combination with
London penetration depth and resistivity measurements, the irradiation was
particularly useful as a phase - sensitive probe of the superconducting order
parameter in iron - based superconductors lending strongest support to sign -
changing $s_{\pm}$ pairing. Here we review the key results on the effect of
electron irradiation in iron-based superconductors.
