By severely reducing the number of molecules in the illuminated sample, the optical resonances of individual molecules can be resolved in a fluorescence excitation spectrum. Photon correlation and sudden jumps due to spectral diffusion confirm the attribution of the Lorentzian excitation peaks to single molecules. This new spectroscopy displays specific effects whith would be washed out in the average over many molecules. A single molecule is also a truly local probe of its environment, by means of which fundamental studies of the matrix dynamics as well as nanophysics experiments may be undertaken.