Our work has yielded new understanding on the nucleation of phases during interdiffusion and interface reaction in thin-film multilayers. With this understanding we have developed the concept of a kinetic bias and have demonstrated the application of biasing to control diffusion pathways and produce phase selection during interfacial reactions in multicomponent systems. These concepts provide for an effective strategy to synthesize structural composites by in-situ reactions and also to develop electronic materials such as photovoltaics or high-temperature devices from multilayers. We have recently extended the capability of using in-situ reactions and kinetic biasing to the design of robust coatings that exhibit self-healing behavior as well as oxidation protection at high temperature.