The recently launched Spitzer Space Telescope (SST: formerly known as SIRTF, the Space Infrared Telescope Facility) is the fourth mission in NASA's Great Observatories program. The observatory, launched on August 25, 2003, is now fully functional, performing well, and is expected to have at least a five year mission. In this colloquium, I will provide an overview of the design, mission and on-orbit performance of the SST, and show data from the results of early release observations. I will also give an overview of the main science themes and observing programs that are being performed by the Spitzer Science Center (SSC), the Guaranteed-Time-Observers and the Legacy Science Teams. Beginning with the Cycle 1 General Observer proposals (due February 14, 2003), the SST will become a powerful new observatory for astronomers to interrogate the universe at mid-to-far infrared (3.5 - 160 microns) wavelength.