The Wilkinson Microwave Anisotropy Probe (WMAP) created a map of temperature fluctuations in the cosmic background radiation -- the remnant afterglow of the hot young universe -- at much higher resolution, sensitivity, and accuracy than its predecessor, the Cosmic Background Explorer (COBE). WMAP observations shed light on several key questions in cosmology, including the structure and physical parameters of the universe. WMAP was launched into orbit in 2001. Observations formally concluded in October 2010, although analysis of the data continues.