Willis Tower is a 110-story skyscraper in Chicago’s Loop that opened in 1973 and, at 1,451 feet, stood as the world’s tallest building for nearly 25 years. Developed as Sears, Roebuck and Co.’s headquarters, it brought together thousands of employees and introduced a distinctive bundled-tube design by architect Bruce Graham and structural engineer Fazlur Rahman Khan, with nine square sections creating its stepped profile. Built after Sears assembled the site in 1970, the tower overcame legal challenges over television interference and remained partly occupied by Sears until 1994. Renamed in 2009, it remains a defining part of Chicago’s skyline and a major attraction through its Skydeck.