  The influence of two important features of magnesium diboride on the
macroscopic transport properties of polycrystalline MgB2 is discussed in the
framework of a percolation model. While two band superconductivity does not
have significant consequences in the field and temperature range of possible
power applications, the opposite is true for the anisotropy of the upper
critical field. The field dependence of the critical current densities strongly
increases and the macroscopic supercurrents disappear well below the apparent
upper critical field. The common scaling laws for the field dependence of the
volume pinning force are altered and Kramer's plot is no longer linear,
although grain boundary pinning dominates in nearly all polycrystalline MgB2
conductors. In contrast to the conventional superconductors NbTi and Nb3Sn, a
significant critical current anisotropy can be induced by the preparation
technique of MgB2 tapes.
