Doped interfaces can have intriguing structures and, in some cases, thermodynamically-stable interfacial complexions can form. In this talk, we explore the usage of complexions in transition metal alloys, with a focus on how these features affect processing routes and mechanical properties. Atomistic simulations are used to identify the effects of chemistry, temperature, and boundary character on grain boundary structural transitions, as well as identify how these features impact plasticity and fracture. Experimental validation is provided by high resolution transmission electron microscopy on specially-designed thin film samples that systematically explore these variables, as well as nanocrystalline alloys produced through a powder route and consolidated to bulk form. As a whole, this work lays the foundation for engineering internal interfaces to design better materials.