We report the use of atom transfer radical polymerization (ATRP) to graft off polymeric microspheres. Residual vinyl groups on nonswellable poly(divinylbenzene-80) microspheres were converted into ATRP initiators by hydroboration/oxidation followed by esterification. Subsequent grafting by ATRP of poly(hydroxylethyl methacrylate) (poly(HEMA)) and poly((dimethylamino)ethyl methacrylate) (poly(DMAEMA)) led to hydrophilically modified microspheres. The surface properties of these microspheres, FT-IR spectra, and potentiometric titration confirm the successful modification. In addition, swellable, lightly cross-linked poly(DVB-co-HEMA) microspheres were modified in a similar manner by grafting, i.e., methyl methacrylate (MMA) and glycidyl methacrylate (GMA), to form graft block copolymers including poly(MMA-b-DMAEMA), poly(MMA-b-trimethylammonium ethylmethacrylate (TMAEMA)), and poly(MMA-b-GMA). Microphase separation of the grafted amphiphilic block copolymers was studied by scanning electron microscopy. The microspheres modified with poly(MMA-b-DMAEMA) and poly(MMA-b-TMAEMA) show polyelectrolyte properties.