Despite their relatively small volume fraction, grain boundaries often play a dominant role in materials proper- ties. Grain boundary engineering was first proposed by Watanabe in 1984 [260], and has since been considered by others. While grain boundary engineering recognizes the effects of grain boundary solute segregation, the emphasis is usually on the geometric structure of grain boundaries and a categorization scheme based on the coin- cident site lattice model that compares “low-angle” and “high-angle” boundaries, or “special” to “general” bound- aries. Clearly, grain boundary complexion transitions deserve attention in the context of grain boundary engi- neering, especially given the non-classical behavior induced by some grain boundary complexion transitions. In partic- ular, grain boundary complexion engineering may provide new opportunities to engineer nano- and microstructures, control mass transport dependent properties and tailor mechanical properties. In this section we will discuss the effect of complexion transitions on grain boundary proper- ties, bulk properties, and materials processing