ISO has shown that the mid–IR spectra of almost all sources are dominated by narrow emission features collectively known as the UIR bands. These bands are carried by a family of polycyclic aromatic hydrocarbon molecules containing about 50 C-atoms. Other classes of molecules such as C-chains as well as non-aromatic groups attached to the PAHs are much less abundant. ISO has revealed that the UIR spectrum is incredibly rich with a large number of weaker features, subfeatures, and shoulders; many of which were not known before. Some examples and the new insights they can provide are described. A systematic study of this spectral structure may well allow us to considerably narrow down the composition of the family of PAHs responsible for the observed emission.