Strong IR emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 μm are a common characteristic of regions of massive star formation in the Milky Way and nearby galaxies and out to redshifts of ~3. These features are carried by large (∼ 50 C-atom) Polycyclic Aromatic Hydrocarbon molecules, which are pumped by the strong FUV photon flux from these stars. ISO/SWS and Spitzer/IRS studies have revealed the richness of the interstellar IR emission spectrum and the variations therein. These variations reflect variations in the physical conditions of the emitting regions.

Over the last 10-20 years, extensive experimental and theoretical studies have elucidated the intrinsic infrared properties of large PAH molecules and their dependence on the molecular characteristics. We are now reaching a stage where we can use the observations of the IR emission spectrum as diagnostic tools to determine the physical conditions in the emitting regions of particular regions of star formation.

Observations have shown that PAH molecules are abundant and ubiquitous in the interstellar medium. Conversely, PAHs may also be a dominant “force” in the interstellar medium. In particular, PAHs dominate the photoelectric heating of interstellar gas and thereby shape the phase (cloud/intercloud) structure of the ISM. PAHs are also the dominant negative charge carriers inside dense molecular cloud cores and, hence, regulate the charge balance of molecular gas. Thus, PAHs control the ambipolar diffusion process and the onset of gravitational collapse.

Because the IR emission features dominate the IR spectrum of regions of massive star formation, these bands are often used as proxies to determine the importance of star formation on galactic scales. Specifically, the importance of star formation versus AGN activity for the luminosity source of ULIRGs is based upon a quantitative interpretation of the observed PAH emission from galactic nuclei.

I will review these different aspects of interstellar IR spectra, the extensive laboratory studies on the spectroscopic and chemical properties of PAHs, their relationship to the intrinsic properties of PAHs, the role of these PAHs in shaping the universe around us – in particular through the star formation process – and the use of PAH emission spectra as quantitative indicators of star formation.