The interface between two immiscible electrolyte solutions (ITIES) plays vital roles in various fields, such as neuroscience, environmental science, analytical chemistry, separation, catalysis, and nanoparticle synthesis. ITIES emerges as a unique approach in chemical analysis due to its sensitivity to both redox-active and redox-inactive analytes. Neurotransmitters, metal ions, drug molecules, and other complex molecules such as proteins have been detected using ITIES. The detection of redox-inactive species at the ITIES has opened a door to understanding a wide range of complex systems that are less suited to probing by solid electrodes. In this feature article, we present the history of electrochemistry at the ITIES; charge transfer reactions, thermodynamics, and kinetics at the ITIES; electrode-solution interfacial structure; and a diverse range of applications enabled by ITIES.