  MIRI (the Mid-Infrared Instrument for the James Webb Space Telescope (JWST))
operates from 5 to 28.5 microns and combines over this range: 1.) unprecedented
sensitivity levels; 2.) sub-arcsec angular resolution; 3.) freedom from
atmospheric interference; 4.) the inherent stability of observing in space; and
5.) a suite of versatile capabilities including imaging, low and medium
resolution spectroscopy (with an integral field unit), and coronagraphy. We
illustrate the potential uses of this unique combination of capabilities with
various science examples: 1.) imaging exoplanets; 2.) transit and eclipse
spectroscopy of exoplanets; 3.) probing the first stages of star and planet
formation, including identifying bioactive molecules; 4.) determining star
formation rates and mass growth as galaxies are assembled; and 5.)
characterizing the youngest massive galaxies. This paper is the introduction to
a series of ten covering all aspects of the instrument.
