Ewine F. van Dishoeck Leiden Observatory, The Netherlands
The bulk of the line and continuum emission of young stellar objects emanates
at mid- and far-infrared wavelengths, and ISO provides the first opportunity
to study the complete spectrum from 2.4 - 197 
m unhindered by the
Earth's atmosphere. The ISO-SWS and LWS spectra of massive young stellar
objects such as Orion BN/IRc2 are reviewed. They contain a wealth of features,
resulting from the disruption of their environment by powerful
outflows and from intense ultraviolet radiation dissociating and ionizing the
surrounding gas. The diagnostic power of the observed lines of H2, CO, OH,
O and H2O will be discussed. At an earlier stage, infrared absorption
spectroscopy toward deeply embedded young stars allows the composition of the
gas and ices in the cold part of the collapse envelope to be probed. Recent
ISO spectroscopic results on circumstellar disks surrounding low- and
intermediate mass stars will be presented. The observations will be discussed
in the context of the physical and chemical evolution of low- and high-mass
young stellar objects.