  The intrinsic properties of MgB$_2$ form the basis for all applications of
this superconductor. We wish to emphasize that the application range of
polycrystalline MgB$_2$ is limited by the upper critical field H$_{c2}$ and its
anisotropy. In wires or tapes, the MgB$_2$ grains are randomly oriented or only
slightly textured and the anisotropy of the upper critical field leads to
different transport properties in different grains, if a magnetic field is
applied and the current transport becomes percolative. The irreversibility line
is caused by the disappearance of a continuous superconducting current path and
not by depinning as in high temperature superconductors. Based on a percolation
model, we demonstrate how changes of the upper critical field and its
anisotropy and how changes of flux pinning will influence the critical currents
of a wire or a tape. These predictions are compared to results of neutron
irradiation experiments, where these parameters were changed systematically.

Superconductivity