  Icy grain mantles are commonly observed through infrared spectroscopy toward
dense clouds, cloud cores, protostellar envelopes and protoplanetary disks. Up
to 80% of the available oxygen, carbon and nitrogen are found in such ices; the
most common ice constituents - H2O, CO2 and CO - are second in abundance only
to H2 in many star forming regions. In addition to being a molecular reservoir,
ice chemistry is responsible for much of the chemical evolution from H2O to
complex, prebiotic molecules. Combining the existing ISO, Spitzer, VLT and Keck
ice data results in a large sample of ice sources (\sime80) that span all
stages of star formation and a large range of protostellar luminosities
(<0.1-105 L\odot). Here we summarize the different techniques that have been
applied to mine this ice data set on information on typical ice compositions in
different environments and what this implies about how ices form and evolve
during star and planet formation. The focus is on how to maximize the use of
empirical constraints from ice observations, followed by the application of
information from experiments and models. This strategy is used to identify ice
bands and to constrain which ices form early during cloud formation, which form
later in the prestellar core and which require protostellar heat and/or UV
radiation to form. The utility of statistical tests, survival analysis and ice
maps is highlighted; the latter directly reveals that the prestellar ice
formation takes place in two phases, associated with H2O and CO ice formation,
respectively, and that most protostellar ice variation can be explained by
differences in the prestellar CO ice formation stage. Finally, special
attention is paid to the difficulty of observing complex ices directly and how
gas observations, experiments and models help in constraining this ice
chemistry stage.
