Determining a diffusion path is not a simple task; for it is the nature of kinetic processes that atomic configurations are arranged in low energy states most of the time. The move from one low energy state to another occurs on timescales usually not accessible to experimental observation. Although the transition path is inaccessible to direct measurement, in some cases it is possible to infer how motion occurs through indirect means. Using atom-tracking STM, in which the tip is "locked" onto the diffusing species using lateral feedback, the diffusion statistics are measured explicitly. >From the diffusion statistics of a dilute surface alloy of palladium or lead impurity atoms embedded in the outermost layer of Cu(001), we determine that the mass transport occurs predominantly via exchange with surface vacancies. Furthermore, the nature of the vacancy-impurity interaction has a dramatic effect on the overall diffusivity. For the case of Pb embedded in the Cu(111) surface, measurements of diffusion on two length scales are necessary to determine the active mechanisms. Short-range motion is predominantly vacancy mediated, while long-range diffusion occurs through exchange with surface adatoms and subsequent transport on top of the surface. Comparison of measurements with first-principles calculations yields valuable insight into diffusion processes, because the energetics of arbitrary diffusion paths can be calculated. Sandia is a Lockheed Martin Company, operated for the U.S. DOE under Contract DE-AC04-94AL85000. This work was supported in part by the Division of Materials Science and Engineering, Office of Science, U.S. DOE. .