This research project proposes to obtain the first unbiased survey on a large flux-limited sample of young stellar objects (YSOs) in 6 different molecular clouds. This study makes use of a huge dataset, namely various photometric maps from 0.6 μm up to 850 μm (including data from the Spitzer and the Herschel Space Telescope), and spectra (between 0.6 and 2.4 μm) of several YSOs in these clouds. The immediate aim of this proposal is to catalogue all the YSOs in the selected star forming regions, getting quantitative information on their accretion and ejection activity. In particular, line luminosities and ratios will be obtained from spectra to measure the fraction of source luminosity due to accretion, and the mass accretion and ejection rates in the late YSOs (Class I, II and III). Spectral energy distributions (SEDs) will be constructed from the photometric data to classify the objects, and to estimate the accretion luminosity and the mass accretion rate for early YSOs (Class 0). Mass ejection rates will be inferred from the YSO outflows.The final goal is then to redefine the YSO evolutionary classification, traditionally based on the SED alone, by means of diagnostic emission lines and the mass accretion rates. This will also allow us to derive the YSO lifetime in the various stages.Additionally, the proposed survey will provide the largest available catalogue of SEDs and spectra from YSOs, which will be fundamental for the selection of promising targets to be observed by highly oversubscribed instrumentation, such as the forthcoming James Webb Space Telescope.