The Wide Field X-Ray Telescope (WFXT) will carry out an unprecedented X-ray survey of galaxy clusters and groups, AGNs and QSOs, and galaxies. WFXT is a medium-class strategic mission that will address key questions in both Cosmic Origins and Physics of the Cosmos. WFXT will be orders of magnitude more effective than previous X-ray missions in performing surveys to a given limiting flux. The angular resolution of ~5" will be finer than provided by any currently planned large-area X-ray survey and highly efficient at discriminating AGNs and QSOs from extended emission from sources such as galaxies and clusters. The Burrows, Burg and Giacconi ideal optical solution gives an approximately constant angular resolution of 3-5 arc seconds across a field of 1-1.5 degrees diameter. A preliminary telescope design provides a resulting grasp an order of magnitude larger than current or future missions. We plan a combination of three surveys and, at each flux limit, WFXT will cover orders of magnitude more area than all previous and planned missions, with the deep 100 deg2 survey reaching the same flux limit as the deepest Chandra surveys to date. The WFXT mission addresses key cosmological and astrophysical science objectives including: the formation and evolution of clusters of galaxies with the associated cosmological and astrophysical implications; black hole formation and evolution; the interaction of black-hole driven AGNs with cluster and galaxy properties; and the high-energy stellar component and the hot ISM phase of galaxies WFXT is a mission for the entire astronomical community. The data from these surveys will be made readily available to the community in timely data releases to be used in a multitude of multi-waveband studies that will revolutionize astronomy.