SWIRE will image about 50 square degrees of high latitude sky in all 7 IRAC and MIPS imaging bands, in 6 of the best wide-area cosmic windows through the Galaxy, providing the widest area tier of Spitzer's suite of extragalactic surveys. SWIRE is optimized to study the evolution of spheroids, spirals, starbursts and AGN within the context of large scale structure, over matched volumes of the Universe to z>2. It will detect about 2.25 million galaxies and provide the largest Spitzer survey volume for the discovery of rare objects.