Metal–support interface often plays dominant roles in supported catalysts. When supported metals are extremely small and consist of single or a few atoms, the effects of metal–support interface are maximized. In such atomically dispersed supported metal catalysts, supports act as ligands and influence the metal–adsorbate and metal–metal interactions significantly. As a result, the structure of supported metal species varies dynamically in response to changes in reaction conditions. Because supported metal species are extremely small, often non-uniform in structure and their structures change dynamically, it is challenging to elucidate the structure–performance relationships of such catalysts. However, efforts to improve precise synthesis methods, atomistic characterization techniques, and theoretical calculations have provided crucial fundamental insights into the structure of active sites, roles of ligands, and effects of neighboring metal atoms. This chapter shows some of the research works aimed to deepen fundamental understanding of the structure–performance relationships of atomically dispersed supported metal catalysts. In addition, to illustrate the importance of such catalysts and prospective opportunities for new catalytic technology that are potentially enabled by them, some recent research works are described.