Sensitive surveys of the X-ray universe have been limited to small areas of the sky due to the intrinsically small field of view of Wolter-I X-ray optics, whose angular resolution degrades with the square of the off axis angle. High angular resolution is needed to achieve a low background per source, minimize source confusion, and distinguish point from extended objects. WFXT consists of three co-aligned wide field X-ray telescopes with a 1° field of view and a 5000 cm2. WFXT will perform three surveys that will cover most of the extragalactic sky to 100-1000 times the sensitivity of the ROSAT All Sky Survey, >~ 2000 deg2 to deep Chandra or XMM-Newton sensitivity, and >~ 100 deg2 to the deepest Chandra sensitivity. WFXT will generate a legacy X-ray data set of >~ 5 x 105 clusters and groups of galaxies to z ~ 2, also characterizing the physics of the intracluster gas for a significant fraction of them, thus providing an unprecedented data set for cosmological applications; it will detect > 107 AGN to z > 6, again obtaining spectra for a substantial fraction; it will detect > 105 normal/starburst galaxies; and it will detect and characterize star formation regions across the Galaxy. WFXT is the only X-ray survey mission that will match, in area and sensitivity, the next generation of wide-area optical, IR and radio surveys. http://wfxt.pha.jhu.edu