Launched from Kennedy Spaceflight Center in the early morning of August 25, 2003, NASA's Spitzer Space Telescope (formerly Space Infrared (IR) Telescope Facility, SIRTF) is the fourth and final facility in the Great Observatories Program. It joins Hubble Space Telescope (HST, 1990), the Compton Gamma-Ray Observatory (CGRO, 1991-2000), and the Chandra X-Ray Observatory (CXO, 1999). Spitzer has a sensitivity that is two to three orders of magnitude higher than that of previous ground-based and space-based infrared observatories. It is revolutionizing our understanding of the creation of the universe, the formation and evolution of galaxies, the genesis of stars and planets, and the chemical evolution of the universe. A brief overview of infrared (IR) astronomy and of Spitzer's role in the science of IR is given. The history, construction, launch, and in-orbit checkout of the observatory is reviewed. Science highlights from the first two and a half years of observations are presented. Further information about the Spitzer can be found on the WEB at http://spitzer.caltech. edu.