Supported catalysts are used in atom transfer radical polymerization (ATRP) to facilitate catalyst recovery and recycling. These catalysts consist of catalytic sites that are covalently tethered to larger supporting particles. It is generally believed that supported ATRP is a surface-mediated polymerization process; that is, both activation and deactivation reactions take place at the surface of the particles. However, recent experiments show that this may not be the case. This paper provides a theoretical analysis to test the concept of surface-mediated ATRP. It is found that deactivation at the surface is unlikely. The geographic isolation of catalytic sites, rather than polymer diffusivity limitation, is primarily responsible for this infeasibility. A trace amount of free catalyst in solution that minimizes the geographic isolation is essential for mediating supported ATRP.