Optical spectroscopy provides a useful tool for probing the structure and dynamics of solids. Over the years, optical methods have evolved from absorption spectroscopy to more complex methods such as spectral hole burning, photon echoes, and luminescence line narrowing. The absorption spectrum provides information about inhomogeneous broadening, while the latter approaches provide information about the systematics of the homogeneous line shape-information that is often obscured by inhomogeneous broadening. When external perturbations such as strain or electric fields are applied, details of the local site can be obtained. Recently, the ultimate limit of optical spectroscopy has been attained, in which the spectral properties of individual single impurity molecules in a solid can be measured, with all ensemble averaging removed. This novel regime has provided additional, previously unobtainable, information on a highly local scale, such as the direct observation of the spectral shifting of a single molecule produced by configurational transitions of the host. It is to be expected that further surprises await us in this new frontier of single-molecule spectroscopy in solids.