We have applied a new method to localize single dye molecules with nanometer resolution in three dimensions by means of the Stark effect. With this method we were able to resolve two molecules which were so close to each other that they underwent a strong dipole‐dipole interaction. A quantitative spectral study of this system allowed us to determine the coupling parameters. In the second part of this paper we discuss our research on metal nanoparticles. We have developed a method to mount single nanoparticles at the very end of a fibre tip. We have used such probes as well defined scattering centers for apertureless scanning near‐field optical microscopy. In this article we also discuss our efforts to examine the interaction of a single metal nanoparticle and a single molecule in a controlled manner. © 2003 American Institute of Physics