Ultrasmall bimetallic nanoclusters (or bi-MNCs for short) have recently emerged as a new class of multi-functional nanoparticles due to their ultrasmall size (typically below 2 nm), unique molecular-like properties (e.g., quantized charging and strong luminescence), controlled cluster compositions (at the atomic level), synergistic physicochemical properties, and rich surface chemistry. Such intriguing properties have motivated the cluster community to develop efficient methods for the synthesis of high-quality bi-MNCs, which can also be seen from the quantum increase of reported synthetic protocols for bi-MNCs. Recent advances in the development of efficient synthesis methods for high-quality bi-MNCs also facilitate the application explorations of bi-MNCs in diverse fields like catalysis, sensors, and biomedicine. This Review article first surveys current progress in the synthesis of bi-MNCs, especially for those NCs with good control of cluster size and composition, followed by a detailed discussion on some unique physicochemical properties of bi-MNCs. The intriguing properties of bi-MNCs have made them ideal platforms for application explorations in catalysis, sensors, and biomedicine, which are discussed in the second section. In the last section, a brief outlook on future developments of functional bi-MNCs is presented, with a particular focus on the controlled synthesis and practical applications of bi-MNCs.