  The Long Wavelength Spectrometer (LWS) on board the Infrared Space
Observatory (ISO) satellite has allowed for the first time to investigate the
whole far infrared (FIR) part of the spectrum (from 43 to 197 um), inaccessible
from the ground. The FIR spectrum is crucial for the study of the early phases
of protostellar evolution since the peak of the continuum emission of the
protostars falls around 100 um and many molecular species emit in this range.
In this paper we review the main results of the spectroscopic observations
carried out with the LWS on a sample of pre-Main Sequence objects in different
evolutionary stages. The FIR spectra are characterized by a strong continuum
with superimposed atomic and molecular emission lines. The fine structure lines
emitted by the OI and CII atoms are present in all the objects of our sample
while emission from molecular species (CO, H_2O and OH) dominate the spectrum
of the younger and more embedded objects; lines of highly ionized atoms (OIII,
NIII, NII) are present only in high luminosity sources. We will show how both
the FIR continuum and the emission lines are powerful indicators of the
evolutionary status of young embedded objects. The analysis of the LWS spectra
has allowed to study only the average properties of the young stellar objects,
due to the large field of view (~ 80 arcsec), the limited sensitivity and the
poor spectral resolution (~ 200) of the instrument in its grating mode. The
higher spatial and spectral resolutions of the instruments on board the
Herschel satellite will allow to address in deeper detail the scientific
problems highlighted by the ISO results.
