Solid-state diffusion is a subject of great interest for many intellectual merits and practical applications. It also provides excellent educational studies with cross-fertilization of science and technology. This paper examines the importance of multicomponent-multiphase interdiffusion with specific examples from materials and coatings for components in advanced energy production systems, including gas turbines and nuclear reactors. Results and analysis from laboratory experiments are presented in terms of interdiffusion fluxes, integrated interdiffusion coefficients, effective interdiffusion coefficients, and average multicomponent interdiffusion coefficients. Applications are highlighted for materials and coatings for components in advanced energy production technologies. Additional consideration is given to the refined approach to assess composition-dependent interdiffusion coefficients in multicomponent alloys.