The spectra of molecules containing more than one atom are necessarily of single atoms. They are correspondingly much more complex than those richer, not only in the number of spectral lines, but also in qualitatively different phenomena which do not have any counterpart in single atoms. Historically, molecular spectra have revealed much fundamental phy­ sics, such as the connection between nuclear spin statistics. They have pro­ vided models of physical systems which have been useful in quite different areas, such as particle physics. Most especially, molecular spectra are of fundamental importance in understanding chemical bonding. They reveal not only bond lengths but also the strength of