  Recent observational studies of intermediate- and high-mass star-forming
regions at submillimeter and infrared wavelengths are reviewed, and chemical
diagnostics of the different physical components associated with young stellar
objects are summarized. Procedures for determining the temperature, density and
abundance profiles in the envelopes are outlined. A detailed study of a set of
infrared-bright massive young stars reveals systematic increases in the
gas/solid ratios, the abundances of evaporated molecules, and the fraction of
heated ices with increasing temperature. Since these diverse phenomena involve
a range of temperatures from < 100 K to 1000 K, the enhanced temperatures must
be communicated to both the inner and outer parts of the envelopes. This
`global heating' plausibly results from the gradual dispersion of the envelopes
with time. Similarities and differences with low-mass YSOs are discussed. The
availability of accurate physical models will allow chemical models of ice
evaporation followed by `hot core' chemistry to be tested in detail.
