Doped interfaces can have intriguing structures and, in some cases, thermodynamically-stable interfacial structures can form.  In this talk, we explore the usage of these “complexions” in nanostructured metal alloys, with a focus on how these features affect processing routes as well as mechanical behavior and resistance to radiation damage.  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, fracture, and point defect recombination.  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.  Microcompression and in-situ bending experiments are used to quantify the effect of doping on mechanical behavior, showing that strength, strain-to-failure, and failure mode can be controlled with the addition of segregating dopants.  As a whole, this work lays the foundation for engineering internal interfaces to design better materials.