SCIENTIFIC ABSTRACT The main objective of the LWS proposals on Pre-Main Sequence evolution is the understanding of the matter evolution from the dilute, cold interstellar state of the dark molecular clouds to the compact, hot stellar state of the Main Sequence stars. The present proposal deals with the study of the stars during their evolution toward the main sequence. The Pre-Main-Sequence contraction phase is generally identified with optically visible young stellar objects of low to intermediate mass, viz. the T Tauri, Herbig Ae/Be stars and the FU Orionis the latter showing outbursts of more than five magnitudes in the optical. All these objects occupy a large part of the Hertzsprung-Russell diagram, with a spread in age ranging from about 10^5 to several 10^7 years. They show experimental evidences of association with disks which are intimately related to the evolution of the central object and to the mass loss phenomenon exibited by most of them. Since most of the disk mass is at relatively low temperatures and the column densities of dust particles through the disk are large, the Pre-MS disks are obvious targets when studying the Pre-MS stellar evolution with ISO. Shock diagnostic emission lines can be used to study the relation between the circumstellar disks and the outflows from the stars. The optical forbidden lines observed in the spectra of these objects (notably [O I] and [S II]) are most probably due to shock excitation by the stellar wind. These shocks produce also a number of strong infrared lines which are affected by extinction to a much lesser extent. Particularly in those cases, where substantial extinction arises in a circumstellar disk, the line shape can be used to map the opacities as function of the wavelength and to estimate the physical scale of the disk. 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 1/20 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 have chosen a few template objects representative of the evolutionary stages addressed in this proposal for FP observations of lines expected to be excited in the different physical regimes.