A crucial problem in star formation is to understand the physical mechanism by which mass is ejected by young stars and collimated into stellar jets and the nature of the link with the accretion process. The interaction of outflowing material with the parental cloud is observable on scales from hundreds of astronomical units (AU) up to parsec scales and is relatively well understood. Conversely, both accretion and ejection take place on very small scales, from a few down to fractions of AU (i.e. milliarcsecond scales for the nearest star forming regions). Thus direct observations of the critical regions has been long hindered by the lack of high angular resolution facilities. The goal of this project is to probe the accretion/ejection region for the first time, by means of very high angular resolution near-infrared interferometric observations (~0.001"). In particular, it is proposed to exploit the unique capabilities of the AMBER instrument at the Very Large Telescope Interferometer to observe the inner region of the accretion disk and the base of the jet down to sub-AU scales in young accreting stars. The analysis of complementary observations at ~0.1" angular resolution will trace the jet collimation/acceleration region and the accretion/ejection interplay on larger scales (tens of AU). These observations, as well as dedicated models of the emission in both permitted and forbidden lines, are crucial for a proper interpretation of interferometric data. The proposed multifaceted approach will provide long-awaited critical observational constraints to test the proposed models of jet launching and magnetospheric accretion, which are central to both star and planet formation. Moreover, the accretion/ejection mechanism is at work in a large range of astrophysical objects from active galactic nuclei to brown dwarfs. Hence, the results of this project will have repercussions for several astrophysical disciplines from extragalactic astronomy to planetary science.