  Currently ~36 different absorption bands have been detected in the infrared
spectra of cold, dense interstellar and circumstellar environments. These are
attributed to the vibrational transitions of ~17 different molecules frozen on
dust grains. We review identification issues and summarize the techniques
required to extract information on the physical and chemical evolution of these
ices. Both laboratory simulations and line of sight studies are essential.
Examples are given for ice bands observed toward high mass protostars, fields
stars and recent work on ices in disks surrounding low mass protostars. A
number of clear trends have emerged in recent years. One prominent ice
component consists of an intimate mixture between H2O, CH3OH and CO2 molecules.
Apparently a stable balance exists between low temperature hydrogenation and
oxidation reactions on grain surfaces. In contrast, an equally prominent ice
component, consisting almost entirely of CO, must have accreted directly from
the gas phase. Thermal processing, i.e. evaporation and crystallization, proves
to be readily traceable in both these ice components. The spectroscopic
signatures of energetic processing by cosmic rays and high energy photons from
nearby protostars are weaker and not as well understood. A fundamental
limitation in detecting complex, energetically produced (and also some simple)
species is blending of weak features in the spectra of protostars.
Sophisticated techniques are required to extract information from blended
features. We conclude with a summary of key goals for future research and
prospects for observations of ices using future instrumentation, including
SIRTF/IRS.
