The emerging single-atom field spans single-atom catalysis in chemistry and single-atom manipulation in physics up to the state-of-the-art characterization via imaging and spectroscopy. These interdisciplinary progresses have been interacted closely with the development of materials science, underscoring the principle that the single atom excels as the smallest functional material. This simple concept not only permits to reinvent our understanding of the nature of materials, but also promises unambiguously to have a great impact on other physical sciences.