  We discuss the temperature, frequency, and power-dependent surface resistance
of the boride superconductor MgB2 in relation to possible applications for
passive microwave devices. The data available in the literature are compared
with results for polycrystalline Nb3Sn and epitaxial YBa2Cu3O7-x, which are
representative of the classical and cuprate superconductors. MgB2 displays all
specific features that make superconductors attractive for high-performance
devices, even though the fabrication technology is not yet mature. We attempt
to identify promising areas of applications, as well as material requirements,
which could further promote the attractiveness of the new superconductor in
this field.
