  Understanding the nature of grain boundary (GB) characteristics in
combination with creating low-energy GBs by modifying the processing
conditions, so-called GB engineering, is of great importance for controlling
and reducing the defect density, leading to improved functionalities of
polycrystalline metals and ceramics. For superconductors particularly,
including both low- and high-temperature superconductors, GB engineering has
been developed to improve especially the critical current densities, Jc, across
these GBs. The intrinsic physical properties of a given superconductor such as
the coherence length, the order parameter symmetry, and their anisotropies
would determine the strategy of GB engineering. In this topical review, we
present an overview of the GB characteristics and GB engineering of Fe-based
superconductors (FBS) in the form of polycrystalline bulks and wires, and thin
films with application potential, e.g. for high-field magnet wires. Prior to
the FBS, GB engineering of the cuprates and MgB2 are also briefly covered.
