<p> The <em>Multi Element Telescope for Imaging and Spectroscopy, METIS</em>, investigation has been conceived to perform off-limb and near-Sun coronagraphy and is motivated by the aim of addressing the three key scientific questions identified as the focus of the HELEX program. These questions concern: the origin and heating/acceleration of the solar wind streams; the origin, acceleration and transport of the solar energetic particles; and the transient ejection of coronal mass and its evolution in the inner heliosphere (coronal mass ejections, CME&rsquo;s).</p> <p> The investigation aims to provide crucial tests apt to verify the hypotheses and models on the following main issues of solar coronal physics that have been developed primarily during the SOHO era:</p> <p> &bull; Is the slow solar wind originating according to more than one physical process and which is the level of contribution of the possible different sources of slow wind to the heliospheric wind observed &lsquo;in situ&rsquo;?</p> <p> &bull; How does the magnetic field topology control the outflow velocity and composition of the slow and fast solar wind?</p> <p> &bull; Is the magnetic field controlling the emergence of the fast wind at the base of coronal holes?</p> <p> &bull; Do polar plumes and inter-plume regions have a role in channeling the fast wind?</p> <p> &bull; Is ion cyclotron dissipation of fast Alfv&eacute;n waves the primary energy deposition process in the fast wind? &bull; Is the reconnection at the base of coronal holes responsible for the generation of waves and turbulence that energize the fast wind?</p> <p> &bull; Which is the nature of coronal heating in open and closed field regions?</p> <p> &bull; Which is the source of the seed particles of the Solar Energetic Particles (SEPs)?</p> <p> &bull; Which is the role of coronal shocks driven by coronal mass ejections in energizing SEPs? &bull; Which are the mass and magnetic fluxes carried out from the Sun during transient events?</p> <p> &bull; How does the evolution of the coronal magnetic field, inferred from the large-scale evolution of the outer corona, trigger coronal mass ejections?</p> <p> &bull; How does the corona re-adjust after coronal mass ejections on medium-term time scales, ranging between 3 and 10 days (not yet established in the outer corona)?</p> <p> &bull; How do the quiescent streamers evolve on time scales ranging between 3 and 10 days? The crucial tests for addressing and solving these still open issues can be achieved by combining the instrument&rsquo;s versatility with the uniqueness of the Solar Orbiter mission profile, which allows: a close approach to the Sun thus leading to a significant improvement in spatial resolution; quasi co-rotation with the Sun, which allows to nearly freeze for several days both the on-disk inner corona and the outer corona in the plane of the sky and, thus, disentangle the evolution of coronal structures and solar rotational effects on medium-term time scales, an outof- ecliptic view of the Sun. METIS instrument is designed to combine and extend the imaging and spectroscopic capabilities of the SOHO and STEREO coronagraphs and spectrometers in order to exploit at &nbsp;</p>