  Disorder - impurities and defects violating an ideal order - is always
present in solids. It can result in interesting and sometimes unexpected
effects in multiband superconductors. Especially if the superconductivity is
unconventional thus having other than the usual s-wave symmetry. This paper
uses the examples of iron-based pnictides and chalcogenides to examine how both
nonmagnetic and magnetic impurities affect superconducting states with $s_\pm$
and $s_{++}$ order parameters. We show that disorder causes the transitions
between $s_\pm$ and $s_{++}$ states and examine observable effects these
transitions can produce.
