The Willis Tower is a 110-story, 1,450-foot (442.1 m) skyscraper in Chicago, Illinois. At completion in 1973, it surpassed the World Trade Center in New York to become the tallest building in the world, a title it held for nearly 25 years; it was the tallest building in the Western Hemisphere for 41 years, until the new One World Trade Center surpassed it in 2014.

Willis Tower is considered a seminal achievement for architect Fazlur Rahman Khan. It is currently the second-tallest building in the United States and the Western hemisphere – and the 16th-tallest in the world. Each year, more than one million people visit its observation deck, the highest in the United States, making it one of Chicago’s most popular tourist destinations.