  The Wide Field X-Ray Telescope (WFXT) is a medium-class mission designed to
be 2-orders-of-magnitude more sensitive than any previous or planned X-ray
mission for large area surveys and to match in sensitivity the next generation
of wide-area optical, IR and radio surveys. Using an innovative wide-field
X-ray optics design, WFXT provides a field of view of 1 square degree (10 times
Chandra) with an angular resolution of 5" (Half Energy Width, HEW) nearly
constant over the entire field of view, and a large collecting area (up to 1
m^2 at 1 keV, > 10x Chandra) over the 0.1-7 keV band. WFXTs low-Earth orbit
also minimizes the particle background. In five years of operation, WFXT will
carry out three extragalactic surveys at unprecedented depth and address
outstanding questions in astrophysics, cosmology and fundamental physics. In
this article, we illustrate the mission concept and the connection between
science requirements and mission parameters.
