• Chicago Skyscraper Willis (Sears) Tower, 1973
  

Although Sears' naming rights expired in 2003, the building continued to be called the Sears Tower for several years. In March 2009, London based insurance broker Willis Group Holdings agreed to lease a portion of the building, and obtained the building's naming rights. On July 16, 2009, the building was officially renamed the Willis Tower. On August 13, 2012, United Airlines announced it will be moving its corporate headquarters from 77 West Wacker Drive to the Willis Tower.

In 1969, Sears, Roebuck & Co. was the largest retailer in the world, with approximately 350,000 employees. Sears executives decided to consolidate the thousands of employees in offices distributed throughout the Chicago area into one building on the western edge of Chicago's Loop. With immediate space demands of 3 million square feet (279,000 m²), and predictions for future growth necessitating more space, Sears commissioned architects Skidmore, Owings and Merrill (SOM) to produce a structure to be one of the largest office buildings in the world. Their team of architect Bruce Graham and structural engineer Fazlur Khan designed the building as nine square "tubes", each essentially a separate building, clustered in a 3×3 matrix forming a square base with 225 foot (75 m) sides. All nine tubes would rise up to the 50th floor of the building. At the 50th floor, the northwest and southeast tubes end, and the remaining seven continue up. At the 66th floor, the northeast and the southwest tubes end. At the 90th floor, the north, east and south tubes end. The remaining west and center tubes continue up to the 108th floor. The building was the first to utilize Khan's bundled tube structure.

Sears executives decided that the space they would immediately occupy should be efficiently designed to house their Merchandise Group. But floor space for future growth would be rented out to smaller firms and businesses until Sears could retake it. Therefore, those floor areas had to be designed to a smaller plate, with a high window space to floor space ratio, to be attractive and marketable to prospective lessees. Smaller floorplates required a taller structure to yield sufficient square footage. Skidmore architects proposed a tower with large 55,000-square foot (5,000 m²) floors in the lower part of the building, and gradually tapered areas of floorplates in a series of setbacks, which would give the Sears Tower its distinctive look.

As Sears continued to offer optimistic projections for growth, the tower's proposed height soared into the low hundreds of floors and surpassed the height of New York's unfinished World Trade Center to become the world's tallest building. Restricted in height by a limit imposed by the Federal Aviation Administration (FAA) to protect air traffic, the Sears Tower was financed by the company. It was topped with two antennas to permit local television and radio broadcasts. Sears and the City of Chicago approved the design, and the first steel was put in place in April 1971. The structure was completed in May 1973. Construction costs totaled approximately US$150 million at the time, equivalent to $790 million in 2013. By comparison, Taipei 101, built in 2004 in Taiwan, cost around the equivalent of US$1.76 billion in 2005 dollars.

Even though regulations didn't require a fire sprinkler system, the building was equipped with one from the beginning. There are about 40,000 sprinkler heads in the building. The sprinkler system cost 4 million dollars.

Black bands appear on the tower around the 29th-32nd, 64th-65th, 88th-89th, and 104th-108th floors. These are louvres which allow ventilation for service equipment and obscure the structure's belt trusses.

In February 1982, two television antennas were added to the structure, increasing its total height to 1,707 feet (520 m). The western antenna was later extended, bringing the overall height to 1,730 feet (527 m) on June 5, 2000 to improve reception of local NBC station WMAQ-TV.

Sears' optimistic growth projections were not met. Competition from its traditional rivals (like Montgomery Ward) continued, with new competition by retailing giants such as Kmart, Kohl's and Wal-Mart. The fortunes of Sears & Roebuck declined in the 1970s as the company lost market share; its management grew more cautious. The Sears Tower was not the draw Sears had hoped it would be. The tower stood half vacant for a decade as a surplus of office space was erected in Chicago in the 1980s.

By 1990, Keck, Mahin & Cate, a law firm, considered moving out of its space in the Sears Tower and moving into a potential new development, which would become 77 West Wacker Drive. Brokers who were familiar with the lease negotiations stated that Sears was trying to keep Keck, Mahin & Cate in the building. Keck, Mahin & Cate decided to move into 77 West Wacker, and the Prime Group, developer of 77 West Wacker, finalized the development of the facility. During the time that Keck, Mahin & Cate was scheduled to move out of the Sears Tower, Sears planned to move its offices to its merchandise group facilities in Hoffman Estates, Illinois. Sears began moving its offices out of the Sears Tower in 1992.

In 1994, Sears sold the building to Boston based AEW Capital Management, with financing from MetLife. At the time, it was one - third vacant. By 1995, Sears had completely vacated the building, moving to a new office campus in Hoffman Estates.

In 1997, Toronto based TrizecHahn Corporation (the owner at the time of the CN Tower) purchased the building for $110 million, and assumption of $4 million in liabilities, and a $734 million mortgage. In 2003, Trizec surrendered the building to lender MetLife.

In 2004, MetLife sold the building to a group of investors, including New York based Joseph Chetrit, Joseph Moinian, Lloyd Goldman, Joseph Cayre and Jeffrey Feil, and Skokie based American Landmark Properties. The quoted price was $840 million, with $825 million held in a mortgage.

In June 2006, seven men were arrested by the FBI and charged with plotting to destroy the tower. Deputy FBI Director John Pistole described their plot as "more aspirational than operational." The case went to court in October 2007; after three trials, five of the suspects were convicted and two were acquitted. The alleged leader of the group, Narseal Batiste, was sentenced to 13½ years in prison in November 2009.

In February 2009, the owners announced they were considering a plan to paint the structure silver. The paint would "rebrand" the building and highlight its advances in energy efficiency. The estimated cost is $50 million.

Since 2007, the building owners have been considering building a hotel on the north side of Jackson, between Wacker and Franklin, at the plaza that is the entrance to the tower's observation deck. The tower's parking garage is beneath the plaza. Building owners say the second building was considered in the original design. The plan was eventually cancelled as city zoning does not permit construction of such a tall tower there.

The Sears Tower observation deck, called the Skydeck, opened on June 22, 1974. Located on the 103rd floor of the tower, it is 1,353 feet (412 m) above ground and is one of the most famous tourist attractions in Chicago. Tourists can experience how the building sways on a windy day. They can see far over the plains of Illinois and across Lake Michigan to Indiana, Michigan and Wisconsin on a clear day. Elevators take tourists to the top in about 60 seconds, and allow tourists to feel the pressure change as they rise up. The Skydeck competes with the John Hancock Center's observation floor a mile and a half away, which is 323 feet (98 m) lower. Some 1.3 million tourists visit the Skydeck annually. A second Skydeck on the 99th floor is also used if the 103rd floor is closed. The tourist entrance can be found on the south side of the building along Jackson Boulevard.

In January 2009, the Willis Tower owners began a major renovation of the Skydeck, to include the installation of retractable glass balconies, extending approximately four feet over Wacker Drive from the 103rd floor. The all glass boxes allow visitors to look through the floor to the street 1,353 feet (412 m) below. The boxes, which can bear five short tons of weight (about 4.5 metric tons), opened to the public on July 2, 2009.

In August 1999, French urban climber Alain "Spiderman" Robert, using only his bare hands and feet, scaled the building's exterior glass and steel wall all the way to the top. A thick fog settled in near the end of his climb, making the last 20 floors of the building's glass and steel slippery.

The building's official address is 233 South Wacker Drive, Chicago, Illinois 60606.

The Willis Tower remains the tallest building in the Americas and the Western Hemisphere. With a pinnacle height of 1729 feet (527 m), it is the second tallest freestanding structure in the Americas, as it is 86 feet (26 m) shorter than Toronto's CN Tower, and is the only other freestanding structure in the Americas to exceed 1500 feet (457.2 m) in height. As of September 2011, the Willis Tower is the seventh tallest freestanding structure in the world (by pinnacle height), after the Burj Khalifa, the Tokyo Skytree, the Abraj Al Bait Towers, the Canton Tower, the CN Tower and the Ostankino Tower.

At 1,482.6 feet (451.9 m) tall, including decorative spires, the Petronas Twin Towers in Kuala Lumpur, Malaysia, laid claim to replacing the Sears Tower as the tallest building in the world in 1998. Not everyone agreed, and in the ensuing controversy four different categories of "tallest building" were created. Of these, Petronas was the tallest in the first category (height to top of architectural elements, meaning spires but not antennas) giving it the title of world's tallest building.

Taipei 101 in Taiwan claimed the record in three of the four categories in 2004 to become recognized as the tallest building in the world. Taipei 101 surpassed the Petronas Twin Towers in spire height and the Sears Tower in roof height and highest occupied floor. The Sears Tower retained one record: its antenna exceeded the Taipei 101's spire in height. In 2008, the Shanghai World Financial Center claimed the records of tallest building by roof and highest occupied floor.

On August 12, 2007, the Burj Khalifa in Dubai, United Arab Emirates, was reported by its developers to have surpassed the Sears Tower in all height categories.

When completed, One World Trade Center in New York City is expected to surpass the Willis Tower through its structural and pinnacle heights, but not by roof, observation deck elevation or highest occupied floor. Burj Khalifa currently holds (by a significant margin) all height records, surpassing the Sears Tower, the CN Tower, One World Trade Center, Taipei 101 and Shanghai World Financial Center in every category. The Chicago Spire, which had a planned height of 610 m (2,000 ft) was expected to lay claim to all categories of height records in the Americas upon completion, but the project was later cancelled in 2010 due to major setbacks.

Until 2000, the Sears Tower did not hold the record for the tallest building by pinnacle height. From 1969 to 1978, this record was held by the John Hancock Center, whose antenna reached a height of 1,500 ft (457.2 m), or 49 ft (14.8 m) taller than the Sears Tower's original height of 1,451 ft (442 m). In 1978, One World Trade Center became taller by pinnacle height due to the addition of a 359 ft (109.3 m) antenna, which brought its total height to 1,727 ft (526.8 m). In 1982, two antennas were installed on top of the Sears Tower which brought its total height to 1,707 ft (520.3 m), making it taller than the John Hancock Center but not One World Trade Center. However, the extension of the Sears Tower's western antenna in June 2000 to 1,730 feet (527 m) allowed it to just barely claim the title of tallest building by pinnacle height.

Although Sears sold the Tower in 1994 and had completely vacated it by 1995, the company retained the naming rights to the building through 2003. The new owners were rebuffed in renaming deals with CDW Corp in 2005 and the U.S. Olympic Committee in 2008. London based insurance broker Willis Group Holdings, Ltd. leased more than 140,000 square feet (13,000 m²) of space on three floors in 2009. A Willis spokesman said the naming rights were obtained as part of the negotiations at no cost to Willis, and the building was renamed the Willis Tower on July 16, 2009. The naming rights are valid for 15 years, so it is possible that the building’s name could change again in 2024. The Chicago Tribune joked that the building’s new name reminded them of the oft repeated "What you talkin' 'bout, Willis?" catchphrase from the 1980s American television sitcom Diff'rent Strokes and considered the name change ill advised in "a city with a deep appreciation of tradition and a healthy ego, where some Chicagoans still mourn the switch from Marshall Field's to Macy's." This feeling was confirmed in a July 16, 2009 CNN article in which some Chicago area residents expressed reluctance to accept the Willis Tower name, and in an article that appeared in the October 2010 issue of Chicago magazine that ranked the building among Chicago's 40 most important, the author pointedly refused to acknowledge the name change and referred to the building as the "Sears Tower". Time magazine called the name change one of the top 10 worst corporate name changes and pointed to negative press coverage by local news outlets and online petitions from angry residents.

The building has appeared in numerous films and television shows set in Chicago such as Ferris Bueller's Day Off, where Ferris and company watch the streets of Chicago from the observation deck. The television show Late Night with Conan O'Brien introduced a character called The Sears Tower Dressed In Sears Clothing when the show visited Chicago in 2006. The building is also featured in History Channel's Life After People, in which it and other human made landmarks suffer from neglect without humans around, and it collapses two hundred years after people are gone. In an episode of the television series, Monk, Adrian Monk tries to conquer his fear of heights by imagining that he is on top of the Sears Tower. Also, in an episode of Kenan and Kel, Kenan Rockmore and Kel Kimble decide to climb to the top of the Sears Tower, so that Kenan can declare his love for a girl.

On May 25, 1981, Dan Goodwin, wearing a homemade Spider - Man suit while using suction cups, camming devices, and sky hooks, and despite several attempts by the Chicago Fire Department to stop him, made the first successful outside ascent of the Sears Tower. Goodwin was arrested at the top after the seven hour climb and charged with trespassing. Goodwin stated the reason he made the climb was to call attention to shortcomings in high-rise rescue and firefighting techniques. After a lengthy interrogation by Chicago's District Attorney and Fire Commissioner, Goodwin was released.

In the movie Category 6: Day of Destruction, the tower is damaged by a tornado.

In "1969", a Season 2 episode of the science fiction series Stargate SG-1, the SG-1 team accidentally travels back in time to the titular year. At one point, the team travels though Chicago and the Sears Tower is shown (erroneously, since construction did not begin on the tower until two years later in 1971).

In the movie I, Robot, the tower is shown updated in the year 2035 with new triangular antennas. The tower is shown surpassed in height by the USR (United States Robotics) Building.

In the 2011 blockbuster, Transformers: Dark of the Moon, the tower is featured in a number of scenes. The most notable one is when the N.E.S.T team tries to enter the city using V-22 Osprey helicopters. They use Willis Tower for cover before using wing suits to descend into the city streets. In the movie, the tower is shown to be severely damaged by the Decepticon invasion of the city.

• The top of the Willis Tower is the highest point in Illinois. The tip of its highest antenna is 1,730 feet (527.3 m) above street level or 2,325 feet (708 m) above sea level, its roof is 1,450 feet 7 inches (442.14 m) above street level or 2,046 feet (623 m) above sea level, the 103rd floor observation deck (The Sky deck) is 1,353 feet (412 m) above street level or 1,948 feet (593 m) above sea level, the Wacker Drive main entrance is 595 feet (181 m) above sea level. (The highest natural point in Illinois is the Charles Mound, at 1,235 feet (376 m) above sea level.)
• The building leans about 4 inches (10 cm) towards the west due to its slightly asymmetrical design, placing unequal loads on its foundation.
• The design for the Willis Tower incorporates nine steel-unit square tubes in a 3 tube by 3 tube arrangement, with each tube having the footprint of 75 × 75 feet (22 × 22 m). The Willis Tower was the first building for which this design was used. The design allows future growth of extra height to the tower if wanted or needed.
• The Franklin Street entrance is 4 feet (1.2 meters) lower than the main entrance on Wacker Drive, for this reason the height of the tower was listed as 1,454 feet (443 meters) for many years, the Council on Tall Buildings and Urban Habitat measures height from the main entrance and thus the official measurement now stands at 1,450 feet (442 meters)
• The restrooms on the 103rd floor 1,353 feet (412 meters) above street level are the highest (relative to street level) in the Western Hemisphere.
• The design was inspired by an advertisement for a package of cigarettes.