Everything you wanted to know about Unidentified Infrared Bands (but did not dare to ask)

Unidentified infrared bands (UIRs) are a family of bands that dominate the IR spectra of a wide variety of objects, including HII regions, PNe, post-AGB objects, YSOs the diffuse ISM, galaxies and galactic nuclei. Understanding these features and the origin and evolution of their carrier has become an important problem in astrophysics: thanks to ISO and Spitzer we now have a better insight of the problem, but still much work has to be done. This talk will give an overview of the UIR problem, starting from the spectral variation issue, through the chemistry of the carriers and possible atronomical applications, toward the first assignement of one these bands.