The aromatic infrared bands (AIBs) centred at 3.3, 6.2, 7.7, 8.6 and 11.2 microns are ubiquitous in diverse interstellar sources including HII regions, reflection nebulae, planetary nebulae, star forming regions, interstellar medium (ISM) of Milky Way as well as external galaxies. Important differences in spectral details have emerged with quality observational data from space based telescopes ISO and SPITZER. The hypothesis that polycyclic aromatic hydrocarbons (PAHs) are the most viable carriers of AIBs was proposed more than two decades back. The infrared (IR) spectra of PAHs is consistent with characteristic frequencies of the AIBs. Improved laboratory and computational spectroscopic techniques have strengthened the hypothesis and PAHs are now considered as important inhabitants of the ISM incorporating substantial fraction of carbon. Theoretical quantum chemical calculations provide an alternative to obtain the IR information of complex species such as substituted and very large and irregular PAHs. The results of theoretical calculations have been used in IR emission models.