Activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) was successfully implemented in aqueous media for the first time. A well-controlled polymerization of oligo(ethylene oxide) methyl ether methacrylate (OEOMA) was conducted with 300 ppm or lower of a copper catalyst and tris(2-pyridylmethyl)-amine (TPMA) ligand in the presence of an excess of halide salts. Ascorbic acid was continuously fed into the reaction mixture to regenerate the activator complex. The effects of the halide salt concentration, ligand concentration, feeding rate of the reducing agent, and copper concentration were systematically studied to identify conditions that provide both an acceptable rate of polymerization and good control over the polymer properties. The optimized polymerization provided linear first-order kinetics, linear evolution of the molecular weight with conversion, and polymers with narrow molecular weight distributions (M-w/M-n < 1.3) at high monomer conversions (similar to 70%) with retention of chain-end functionality. The reaction rate could be directly controlled by stopping or starting the continuous feeding of the reducing agent. Finally, the aqueous ARGET ATRP technique was applied to biological systems by synthesizing a well-defined protein-polymer hybrid by the "grafting from" method.