The self-assembly of nanoparticles (NPs) at liquid-liquid interfaces (LLIs) has recently emerged as a promising platform for tunable optical devices, sensors, and catalysis. There are numerous advantages for such platforms when compared to more conventional solid-state counterparts. For example, they do not need engineering, self-assemble if proper conditions are provided, are self-healing, are practically nondegrading, and are easily renewable. Furthermore, they have the added benefit of being able to facilitate the interactions of analytes dissolved in often-inaccessible environments. In this Perspective, we highlight some important recent developments in understanding the mechanisms and applications of self-assembly of NPs at LLIs for use as mirrors and sensors. Finally, we explore future directions in this field, focusing on NP arrays with electrotunable properties assembled at a LLI, which has been one of the driving forces for developing such technologies.