Light is able to excite the free electrons of a metal to collective oscillations. In noble metal nanoparticles of 10-100 nm diameter this leads to a resonance in the visible spectral range, the so-called plasmon resonance. One finds an enhanced electro-magnetic field in the environment of the nanoparticle. We use this field enhancement to investigate weak optical effects on the nanoscale. Plasmonic nanoparticles can thus be employed as optical nanoantennas. A moving antenna is the idea of a scanning optical nearfield microscope. We develop and use such a microscope to investigate ultrafast processes with a spatial resolution of only a few nanometers.