Mid-infrared astronomy from the ground is severely limited by huge thermal backgrounds. Sensitive measurements have depended on space missions, which have been limited in aperture and hence in angular resolution. For the first time, MIRI with JWST provides:  1.) fantastic sensitivity; 2.) high angular resolution; 3.) no atmospheric interference; 4.) high stability operation in space; and 5.) a versatile set of measurement capabilities. I will introduce the instrument design and capabilities. I will then illustrate its potential with some science examples: 1.) identifying "first light" galaxies; 2.) probing active galactic nucleus evolution; 3.) identifying biologically important molecules around forming stars; and 4.) determining the atmospheric compositions and thermal balance of exoplanets.