  In this short review, we attempt to give a comprehensive discussion of
studies performed to date by muon-spin spectroscopy (more precisely the
relaxation and rotation technique, also know as \muSR technique) on the
recently discovered layered iron-based superconductors. On one side, \muSR has
been used to characterized the magnetic state of different families of layered
iron-based systems. Similarly the subtle interplay of the magnetic state and
the structural transition present in some families has been investigated. We
will also discuss the information provided by this technique on the interaction
between the magnetic state and the superconducting phase. Finally the \muSR
technique has been used to investigate the magnetic penetration depth of the
superconducting ground state. The study of its absolute value, temperature and
magnetic field dependence provides crucial tests for investigating possible
unconventional superconducting states in such systems.
