Polymer-modified microemulsions can be obtained by adding polymers to a mixture consisting of water, oil, and a cosurfactant. In dependence on the type of polymer used quite different effects can be observed. On the one hand water soluble polymers can be incorporated into the individual water droplets of a water-in-oil microemulsion, or into droplet clusters, and on the other hand they can induce the formation of a sponge phase, that means a bicontinuous microemulsion. Our experiments have shown that nonionic polymers, hydrophobically modified polyampholytes, as well as polycations can induce the formation of a phase channel between the water-in-oil and the oil-in-water microemulsion. The polymer-modified microemulsions can be used as a new type of template for the formation of nanoparticles. Therefore, polymer-stabilized barium sulfate nanoparticles of quite different size and shape can be formed in the microemulsion template phase, and redispersed after solvent evaporation in water.