MIDIR is a combined thermal/mid-infrared imager and spectrograph for the European Extremely Large Telescope (EELT). It will operate in the infrared L, M, N, and Q-band to 20μm with a goal to extend the wavelength coverage to 27μm if the atmospheric properties of the site are sufficiently good. MIDIR will offer imaging and spectroscopic modes over a wide range in spectral resolution. MIDIR will be designed for diffraction limited performance, requiring an optimized, cryogenic adaptive optics (AO) system. The conceptual study of MIDIR is part of a suite of eight ELT instrument "small studies" partly funded by the EU . The study is being performed by an international consortium of Leiden Observatory, Astron, MPIA, UK-ATC, and ESO. The high level instrument requirements for MIDIR have been directly derived from numerous important science cases. In this paper we discuss the science case for MIDIR, provide a summary of the technical specifications, discuss the requirements on the AO system, and estimate the sensitivity in various observing modes. More technical details on the instrument are given in a parallel paper at this conference .