  The upper critical field and flux pinning in MgB2 single crystals were
investigated. The implications of these properties for technical applications
are discussed and compared with transport properties of polycrystalline bulk
samples and wires. In these untextured materials current percolation is
important, especially at high magnetic fields. It is shown that the anisotropy
of the upper critical field influences the "irreversibility line" and that the
application range of MgB2 is limited by the smallest upper critical field
(i.e., for the field direction perpendicular to the boron planes). Disorder,
introduced by irradiation with neutrons, enhances the upper critical field,
reduces the anisotropy and drastically changes flux pinning. While the enhanced
Hc2 and the reduced anisotropy generally improve the transport properties of
the polycrystalline samples, the contribution of the radiation-induced defects
to flux pinning is small compared to the as-grown defect structure (grain
boundary pinning).
