  Tunneling data on magnesium diboride, MgB_2, are reviewed with a particular
focus on superconductor-insulator-superconductor (SIS) junctions formed by a
break-junction method. The collective tunneling literature reveals two distinct
energy scales, a large gap, Delta_L~7.2 meV, close to the expected BCS value,
and a small gap, Delta_S~2.4 meV. The SIS break junctions show clearly that the
small gap closes near the bulk critical temperature, T_c=39 K. The SIS spectra
allow proximity effects to be ruled out as the cause for the small gap and
therefore make a strong case that MgB_2 is a coupled, two-band superconductor.
While the break junctions sometimes reveal parallel contributions to the
conductance from both bands, it is more often found that Delta_S dominates the
spectra. In these cases, a subtle feature is observed near Delta_S+Delta_L that
is reminiscent of strong-coupling effects. This feature is consistent with
quasiparticle scattering contributions to the interband coupling which provides
an important insight into the nature of two-band superconductivity in MgB_2.
