Characterization of grain boundaries (GBs) using EBSD is not only efficient but also provides many quantitative parameters of GB microstructure. By comparing a measured misorientation with already developed models of GBs, such as the coincidence site lattice (CSL) model, characters defined by the models can be assigned to that misorientation, and hence grain boundary character distribution (GBCD) in terms of length fraction or number fraction can be easily obtained. EBSD characterization has dramatically facilitated the understanding of the relationship between grain boundary characters and material properties. This chapter reviewed the authors’ works of using EBSD to determine grain boundary character for investigation of twinning-induced grain boundary engineering (GBE) of stainless steels and nickel-based alloys. The formation of large-size highly-twinned grain-clusters (also termed as twin related domain) is the marked feature of a GBE microstructure. The higher-order twin relationships between any two grains within the cluster, whether adjacent or not, can be directly determined through twin-chain investigation, with the help of which the mistakes in identifying pseudo random GBs and non-twin type CSL GBs could be avoided. The highly-twinned grain-cluster is developed by extensive multiple twinning stating from a single recrystallization nucleus, therefore low nucleation density of recrystallization as a result of low-level of pre-stain and subsequent high temperature annealing is essential for achieving a GBE microstructure. The low Σ CSL GBs interrupting random boundaries among grain-clusters have a larger deviation than those inside the clusters, indicting these low Σ CSL GBs simply happen to fall in the tolerances of the Brandon criterion. The 3D-EBSD characterization shows that GBE specimen has more large-area morphologically-complex twin boundaries, and hence has a higher area fraction but similar number fraction of twin boundaries as compared with conventional specimen. The precipitation of carbides and Cr depletion can be greatly suppressed at coherent twin boundaries, which is the reason for a much better resistance to intergranular corrosion, therefore only this type of GB is special. Intergranular attack penetration would be arrested at Σ3–Σ3–Σ9 triple junctions once it enters a highly-twinned grain-cluster, resulting in a better IGC resistance of a GBE microstructure than a conventional one.