The Spitzer Space Telescope, launched in August 2003, is the infrared member of NASA's Great Observatory family. Spitzer combines the intrinsic sensitivity of a cryogenic telescope in space with the imaging and spectroscopic power of modern infrared detector arrays. This review covers early results from Spitzer that have produced major advances in our understanding of our own solar system and phenomena within the Galaxy. Spitzer has made the first detection of light from extrasolar planets, characterized planet-forming and planetary debris disks around solar-type stars, showed that substellar objects with masses smaller than 10 M Jup form through the same processes as do solar-mass stars, and studied in detail the composition of cometary ejecta in our Solar System. Spitzer's major technical advances will pave the way for yet more powerful future instruments. Spitzer should operate with full capabilities well into 2009, enabling several additional cycles of discovery and follow-up.