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, Cu^I is the major activator of alkyl halides, Cu⁰ is a supplemental activator and reducing agent of excess Cu^II through comproportionation, and disproportionation is negligible. In contrast, in SET-LRP, Cu⁰ is the major activator, Cu^I 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.