An elementary process occurring on surfaces is diffusion. The dynamics is simplest when the concentration of adsorbates is sufficiently small that interaction between adsorbates can be ignored. But even for this tracer diffusion process, much remains to be uncovered. Here, we present the interplay between experimental measurement of tracer diffusion and its theoretical interpretation, which leads to good estimates of the interaction of the adparticle with the surface. We show how the results from three different experimental techniques—field ion microscopy, scanning tunnelling microscopy and quasielastic helium atom scattering—can be interpreted. Using the generalized Langevin equation as a model for the diffusion dynamics, we show how the turnover theory for activated diffusion may be used to describe the measured time evolution of the adparticle distribution on the surface. The different activation energy measured for hopping over single or double lattice lengths is shown to come from the added energy loss to the surface, as the particle moves over the longer path. We discuss some of the issues which are not yet clear; these include quantum effects, such as the quantum suppression of diffusion, vibrationally assisted diffusion, multidimensional effects and diffusion in the presence of external fields.