A debate has proceeded in the literature regarding the mechanism of reversible-deactivation radical polymerization in the presence of Cu0. The two proposed models are supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) and single electron transfer living radical polymerization (SET-LRP). In SARA ATRP, CuI is the major activator of alkyl halides, Cu0 is a supplemental activator and reducing agent of excess CuII through comproportionation, and disproportionation is negligible. In contrast, in SET-LRP, Cu0 is the major activator, CuI does not activate alkyl halides but instead undergoes instantaneous disproportionation. In this article we present a body of experimental and theoretical data, which supports the SARA ATRP mechanism and disagrees with the SET-LRP mechanism.