  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.
