Aqueous polymer dispersions useful for coating substrates are prepared by polymerization of olefinically unsaturated monomers in aqueous medium in the presence of radical formers and hydrophilically modified, urethane-group-containing emulsifiers having an average molecular weight below 30,000, wherein the urethane-group-containing emulsifiers are oligourethanes having a branched molecular structure which(i) are produced from organic polyisocyanates and organic compounds containing isocyanate-reactive groups with the reactants containing hydrophilic groups or groups convertible into hydrophilic groups and which are monofunctional or difunctional to isocyanate addition reaction, with the molar ratio of monofunctional hydrophilic components to the corresponding difunctional components being at least 2:1, and which(ii) have a content of chemically incorporated hydrophilic groups sufficient to impart the oligourethane solubility or dispersibility in water, with the proviso that, on a statistical average, at least 2.2 terminally incorporated hydrophilic groups are present per molecule.