  Seven years after the discovery of superconductivity in magnesium diboride,
the fundamental superconducting properties of this compound are well known and
the peculiar current transport in polycrystalline materials is essentially
understood. Based on this knowledge the ultimate performance of wires or tapes
at high magnetic fields will be predicted and compared to state-of-the-art
materials and to other superconductors. The key parameter for high field
applications is the upper critical field, which can be strongly enhanced by
impurity scattering. This fundamental property might be further optimized in
bulk materials, since higher values were reported for thin films. The MgB$_2$
grains are usually very small, if prepared by the in-situ technique. The
resulting high density of grain boundaries leads to strong pinning, close to
the theoretical limit. On the other hand, the connectivity between the grains
is still rather poor and strongly reduces the achievable critical currents,
thus leaving room for further improvements.
