This article critically assesses the current status and future directions for the development of interfacial phase diagrams for applications in activated sintering and other fields. The origin of solid-state activated sintering is attributed to the enhanced mass transport in sintering-aid-based, nanoscale, quasi-liquid, interfacial films that are stabilized below the bulk solidus line. Interfacial thermodynamic models have been developed via extending a phenomenological premelting theory and incorporating the computational thermodynamic (CalPhaD) methods. A primitive type of interfacial phase diagrams, ?-diagrams, have been computed, and these diagrams have been validated by experiments and proven useful. More rigorous interfacial phase diagrams with well-defined transition lines and critical points may also be constructed. A long-range scientific goal is proposed to develop interfacial phase diagrams as a new materials science tool. Future studies should be conducted in several areas to achieve this goal, and special efforts should be made to predict the complex interfacial phase behaviors in multicomponent ceramic materials. Potential broad applications are envisaged.