  The Fe-based superconductors (FBS) are an important new class of
superconducting materials. As with any new superconductor with a high
transition temperature and upper critical field, there is a need to establish
what their applications potential might be. Applications require high critical
current densities, so the usefulness of any new superconductor is determined
both by the capability to develop strong vortex pinning and by the absence or
ability to overcome any strong current-limiting mechanisms of which grain
boundaries in the cuprates are a cautionary example. In this review we first
consider the positive role that grain boundary properties play in the metallic,
low temperature superconductors and then review the theoretical background and
current experimental data relating to the properties of grain boundaries in FBS
polycrystals, bi-crystal thin films, and wires. Based on this evidence, we
conclude that grain boundaries in FBS are weak linked in a qualitatively
similar way to grain boundaries in the cuprate superconductors, but also that
the effects are a little less marked. Initial experiments with the textured
substrates used for cuprate coated conductors show similar benefit for the
critical current density of FBS thin films too. We also note that the
particular richness of the pairing symmetry and the multiband parent state in
FBS may provide opportunities for grain boundary modification as a better
understanding of their pairing state and grain boundary properties are
developed.
