This chapter provides information on cluster surface diffusion, adsorbate-modified surface self-diffusion and concentration dependence in surface hetero-diffusion. Adsorbed particles diffusing on a crystalline surface interact with each other and can occasionally form clusters. The correlated motion of such clusters, are formed in the FIM by depositing atoms from the vapor phase. For channelled surfaces the orientation of the cluster relative to the direction of the channel is important. The mobility of larger clusters is generally lower than that of dimers (pairs). The activation energies increase with increasing size of the cluster. The chemical composition of a metal surface, i.e. the exact knowledge of impurities on or near the surface, plays a very important role for the rate of surface self-diffusion. Impurity effects may also occur for hetero-diffusion but these are less well studied. In all cases one should distinguish results in which impurities have been intentionally introduced on the surface from those where unintentionally added, often unidentified impurities cause deviations from measurements on clean surfaces. Very large surface diffusion coefficients above 10-7m2/s are observed so that the diffusion coefficients are characteristic of partially molten surface layers. For surface diffusion of adsorbed hetero-species in the sub-monolayer to monolayer coverage range one observes often a strong coverage dependence of surface diffusion coefficients. The reason for this behavior lies in particle interactions that may even lead to various ordered phases at certain coverages and temperatures. The coverage dependence of surface hetero-diffusion is a general phenomenon.