The Willis Tower, formerly (1973–2009) Sears Tower, is a skyscraper office building in Chicago, Illinois; it is one of the world’s tallest buildings. The Sears Tower opened to tenants in 1973, though construction was not actually completed until 1974. In 2009 the London-based insurance broker Willis Group Holdings (later called Willis Towers Watson) leased more than 140,000 square feet of office space on three floors of the Sears Tower. The contract included naming rights for 15 years. On July 16, 2009, the name of the building was officially changed to Willis Tower.