The Infrared Space Observatory (ISO) has provided the first complete mid-IR spectra for a wide range of objects. Almost all of these spectra are dominated by the well-known infrared emission features at 3.3, 6.2, 7.7, and 11.2 $mu$m, the so-called Unidentified Infra-Red (UIR) features. Besides the major features, there is an array of minor features and broad plateaux stretching from 3 to 20 $mu$m which reveal subtle details of conditions in the emission zones and properties of the carriers. Generally attributed to the vibrational relaxation of UV-pumped Polycyclic Aromatic Hydrocarbon molecules (PAHs) containing some 50--100 C-atoms, these UIR spectra are a treasure trove of information. The ISO spectra have, for the first time, allowed a systematic analysis of the spectral characteristics of the UIR features in a wide variety of environments. The peak positions, profiles, and relative strengths of the major features vary from source to source and spatially within sources. These specific profiles are not unique to certain object types but can occur within each individual source. Here, we review ISO and recent ground-based observations and assess some of their implications.
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