  METIS, the Mid-infrared ELT Imager and Spectrograph (formerly called MIDIR),
is a proposed instrument for the European Extremely Large Telescope (E-ELT),
currently undergoing a phase-A study. The study is carried out within the
framework of the ESO-sponsored E-ELT instrumentation studies. METIS will be
designed to cover the E-ELT science needs at wavelengths longward of 3um, where
the thermal background requires different operating schemes. In this paper we
discuss the main science drivers from which the instrument baseline has been
derived. Specific emphasis has been given to observations that require very
high spatial and spectral resolution, which can only be achieved with a
ground-based ELT. We also discuss the challenging aspects of background
suppression techniques, adaptive optics in the mid-IR, and telescope site
considerations. The METIS instrument baseline includes imaging and spectroscopy
at the atmospheric L, M, and N bands with a possible extension to Q band
imaging. Both coronagraphy and polarimetry are also being considered. However,
we note that the concept is still not yet fully consolidated. The METIS studies
are being performed by an international consortium with institutes from the
Netherlands, Germany, France, United Kingdom, and Belgium.
