Single molecule spectroscopy (SMS) is optical spectroscopy with ultimate sensitivity, because sample for SMS is just one molecule (particle, complex, aggregate ect). Today it allows studying fluorescence, Raman scattering and even absorption signals coming from a single molecule using a powerful set of optical spectroscopy techniques. A common approach to select only one molecule for observation is to have so low concentration of them on a surface that the distances between the neighboring molecules are substantially larger than the diffraction limit. Then individual fluorescing molecules can be observed like “stars on the sky” in a fluorescence microscope (see the photo above). SMS allows studying phenomena that, although present as underlying dynamics, are not observable for bulk materials at all due to ensemble averaging. The most well-known effect of this kind is fluorescence “blinking” - strong fluctuations of fluorescence intensity, polarization, or spectra of a single molecule or nanoparticle. Individual nano objects can be used a local sensor of the environment and as a source of single photons for quantum computing and cryptography. SMS methods applied to semiconductor nanoparticles, large conjugated polymer molecules, light harvesting antenna complexes and other systems at the nano scale makes it possible to understand fundamental physics in these systems which otherwise blurred or even hidden by inhomogeneity often inherent to such nano materials.