SCIENTIFIC ABSTRACT The main objective of the LWS proposals on Pre-Main Sequence evolution is the understanding of the matter evolution from the molecular clouds to the compact, hot stellar state of the main sequence stars. The present proposal deals with the study of the peculiar phase during which powerful, collimated bipolar outflows are originated from the central object. A consistent picture of the early phases of stellar evolution has to include the understanding of the outflow phenomena and its relationship to the central exciting source and its circumstellar disk. It has been realized that in order to understand the young stars we have to understand their circumstellar disks as these contribute significantly to the evolution and the energy output from these objects. In particular it has been enpirically estabished that the physics of these disks is intimately related to the physics of the stellar mass loss and possibly to the physics of the mass accretion as well. The LWS will be used to study the ionization and excitation state of the warm outflowing gas, to determine the velocity law by comparing the observed line intensities with models and mapping the dust opacity in the circumstellar disks measuring the line profiles as a function of the wavelength. Both spectrometers will be used in low resolution mode, in order to detect the various spectral features expected to trace the different physical conditions. We expect to detect line emission from various ionic and molecular species (in particular H_2O, OI, CO, NII that are among the major coolants in th e expected physical conditions). OBSERVATION SUMMARY Full spectral scans using the LWS in grating mode are planned for all our targets. The "on source" integration time for LWS Grating spectra are computed in order to observe with S/N=10 lines of 2 10^-20 W/cm^2. For the brightest sources this limit is reduced in order to observe with S/N=10 lines that are 1/200 of their 60 um continuum. In presence of an "outflow" the "off source" observation is done carrying out a map (in "raster scan") of the flow with a full grating spectrum in each point of the map. The total integration time spent on all the points of the map will be similar to the one spent on the single "on source" point; the reference "off source" point will be therefore built by binning together all the points of the map. A full SWS grating scan is foreseen for all the objects (with the exception of the objects that will be observed in the SWS guaranteed time) with a 25 um IRAS flux sufficient to give S/N=10 on a line that is 1/30 of the continuum in the SWSO1 mode. We also scan with SWS01 mode those points in the flow where evidence of near-infrared shock excited emission exists.