I. INTRODUCTION A series of experiments in the late 1960s and in the 1970s discovered that molecular fluorescence and Raman scattering could be enhanced close to metal surfaces $^{1–5}$. These pio- neering efforts spurred a fury of theoretical works in the subsequent years to understand and explain the phenomena encountered in surface-enhanced spectroscopy $^{6–10}$. Among various effects, these efforts identified field enhancement at sharp edges and surface roughness as a source of stronger fluorescence excitation and emission. It was also predicted that plas- mon resonances in metallic films and nanostructures could enhance these effects further. Moreover, fluorescence quenching and the modification of the emission quantum yield due to the absorption in real metals were investigated theoretically. For all these effects, one discovered a very strong influence on the emitter orientation and position with respect to the metallic structure under study. These steep dependencies, which are inherent to near- field interactions, posed a great challenge to the experimental verification and quantitative understanding of surface-enhanced phenomena because they could not be controlled in en- semble measurements. As a result, only the large enhancement factors had to be deduced from averaging measurements. The advent of scanning near-field optical microscopy (SNOM) 11,12 and single molecule spectroscopy 13,14 in the 1980s and their maturation in the 1990s provided the experimental tools for controlled laboratory investigation of surface-enhanced interactions. In 2004 we succeeded, for the first time, to examine the enhancement of fluorescence from a single oriented molecule as a function of its precise three-dimensional (3D) location close to a single gold nanoparticle. To do this, we placed a single gold nanosphere at the end of a glass fiber tip 15 and used scanning probe microscopy to position it in the near field of single molecules embedded in a thin film. In an extended series of measurements we investigated the shortening of the fluorescence lifetime, the modification of the emission spectrum, the change in the emission pattern, dependence of the excitation rate on the illumination wavelength within the plasmon resonance, and the size of the gold nanoparticle $^{16–19}$. These studies showed that a spherical gold particle acts as a nanoantenna that modifies the excitation rate, the spontaneous emission rate, the emission spectrum, and the radiation pattern of an emitter in its near field. Indeed, several other groups have also investigated single molecule fluorescence in the near field of well-defined scanning probes, which function as optical 2