The Wide-field Infrared Survey Explorer (WISE) is a NASA Medium Explorer (MIDEX) currently surveying the entire sky in 4 mid-infrared bands at 3.4, 4.6, 12 and 22 microns with vastly greater sensitivity than previous all-sky surveys at these wavelengths. WISE observes everything that is further from the Sun than the Earth, and this includes minor planets, comets, nearby brown dwarfs and star forming regions both in the Milky Way and in distant galaxies. The WISE long wavelength channels are very powerful for detecting Ultra-Luminous Infrared Galaxies, and WISE should detect the most luminous galaxies in the Universe. The WISE short wavelength channels are very powerful for detecting old cold brown dwarfs, and WISE should detect the nearest brown dwarfs to the Sun. WISE will also measure the radiometric diameters of about 200,000 asteroids. WISE has a 40 cm cryogenic telescope, 1024x1024 arrays, a scan mirror to freeze images on the arrays while the spacecraft scans continuously, and takes 47'x47' images every 11 seconds in all four bands from an IRAS/COBE style Sun-synchronous nearly polar low Earth orbit. WISE launched from Vandenberg Air Force Base on 14 Dec 2009, ejected its cover on 29 Dec 2009, and entered routine survey operations on 14 Jan 2010. By May 30 WISE had taken over a million four-band images.