The behavior of individual atoms on solids has long been of interest for understanding the physical and chemical properties of surfaces. Now, through the use of the field ion microscope, it is possible to directly image single atoms on metal surfaces and to examine their properties quantitatively. Such studies are reviewed, with special emphasis on: (1) surface structure and its effect on diffusion; (2) the details of the jump process; (3) interactions between atoms leading to cluster formation; and (4) the properties of clusters.