The spectroscopy of single molecules in frozen matrices at liquid helium temperatures reveals very narrow lines, often with lifetime-limited widths. The sensitivity of such sharp lines provides a wealth of information about the molecules, their environment, and their interaction with electromagnetic fields. As compared to more conventional bulk investigations, single molecules reveal the full extent of inhomogeneity at small scales, in the static or structural sense as well as in the time-resolved or dynamical sense. A few examples from the recent literature, on single molecules as sources of single photons, as probes for molecular or electronic motion, or as beacons in structural studies of biological molecules, illustrate these general features of the technique. The author's best hopes for advances in the field include bridging between room and liquid helium temperatures, investigating structural problems on frozen biomolecules, using a single molecule as a tip for near-field optics, or as an input or output gate for quantum cryptography and quantum computing, and probing and exploiting the multiple possible interactions between single molecules.