The Leaning Tower of Pisa (Italian: Torre pendente di Pisa) or simply the Tower of Pisa (Torre di Pisa) is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa. It is situated behind the Cathedral and is the third oldest structure in Pisa's Cathedral Square (Piazza del Duomo) after the Cathedral and the Baptistry.

The height of the tower is 55.86 m (183.27 ft) from the ground on the low side and 56.70 m (186.02 ft) on the high side. The width of the walls at the base is 4.09 m (13.42 ft) and at the top 2.48 m (8.14 ft). Its weight is estimated at 14,500 metric tons (16,000 short tons). The tower has 296 or 294 steps; the seventh floor has two fewer steps on the north facing staircase. Prior to restoration work performed between 1990 and 2001, the tower leaned at an angle of 5.5 degrees, but the tower now leans at about 3.99 degrees. This means that the top of the tower is displaced horizontally 3.9 metres (12 ft 10 in) from where it would be if the structure were perfectly vertical.

A popular tourist activity is to pose for photographs pretending to "hold up" the leaning tower and preventing it from falling. The illusion is created through the principle of forced perspective.

Construction of the tower occurred in three stages across 177 years. Work on the ground floor of the white marble campanile began on August 8, 1173, during a period of military success and prosperity. This ground floor is a blind arcade articulated by engaged columns with classical Corinthian capitals.

The tower began to sink after construction had progressed to the second floor in 1178. This was due to a mere three metre foundation, set in weak, unstable subsoil, a design that was flawed from the beginning. Construction was subsequently halted for almost a century, because the Republic of Pisa was almost continually engaged in battles with Genoa, Lucca and Florence. This allowed time for the underlying soil to settle. Otherwise, the tower would almost certainly have toppled. In 1198 clocks were temporarily installed on the third floor of the unfinished construction.

In 1272 construction resumed under Giovanni di Simone, architect of the Camposanto. In an effort to compensate for the tilt, the engineers built upper floors with one side taller than the other. Because of this, the tower is actually curved. Construction was halted again in 1284, when the Pisans were defeated by the Genoans in the Battle of Meloria.

The seventh floor was completed in 1319. It was built by Tommaso di Andrea Pisano, who succeeded in harmonizing the Gothic elements of the bell chamber with the Romanesque style of the tower. There are seven bells, one for each note of the musical major scale. The largest one was installed in 1655. The bell chamber was finally added in 1372.

After a phase (1990 – 2001) of structural strengthening, the tower is currently undergoing gradual surface restoration, in order to repair visual damage, mostly corrosion and blackening. These are particularly pronounced due to the tower's age and its exposure to wind and rain.

• On January 5, 1172, Donna Berta di Bernardo, a widow and resident of the house of dell' Opera di Santa Maria, bequeathed sixty soldi to the Opera Campanilis petrarum Sancte Marie. The sum was then used toward the purchase of a few stones which still form the base of the bell tower.
• On August 9, 1173, the foundations of the Tower were laid.
• Nearly four centuries later Giorgio Vasari wrote : "Guglielmo, according to what is being said, in [this] year 1174 with Bonanno as sculptor, laid the foundations of the belltower of the cathedral in Pisa."

• Another possible builder is Gerardo di Gerardo. His name appears as a witness to the above legacy of Berta di Bernardo as "Master Gerardo", and as a worker whose name was Gerardo.
• A more probable builder is Diotisalvi, because of the construction period and the structure's affinities with other buildings in Pisa. But he usually signed his works, and there is no signature by him in the belltower.
• Giovanni di Simone was heavily involved in the completion of the tower, under the direction of Giovanni Pisano, who at the time was master builder of the Opera di Santa Maria Maggiore. He could be the same Giovanni Pisano who completed the belfry tower.
• Giorgio Vasari indicates that Tommaso di Andrea Pisano was the designer of the belfry between 1360 and 1370.
• On December 27, 1233, the worker Benenato, son of Gerardo Bottici, oversaw the continuation of the construction of the belltower.
• On February 23, 1260, Guido Speziale, son of Giovanni, a worker on the cathedral Santa Maria Maggiore, was elected to oversee the building of the Tower.
• On April 12, 1264, the master builder Giovanni di Simone and 23 workers went to the mountains close to Pisa to cut marble. The cut stones were given to Rainaldo Speziale, worker of St. Francesco.

Galileo Galilei is said to have dropped two cannon balls of different masses from the tower to demonstrate that their speed of descent was independent of their mass. However, this is considered an apocryphal tale, its only source being Galileo's secretary.

During World War II, the Allies discovered that the Germans were using the tower as an observation post. A U.S. Army sergeant was briefly entrusted with the fate of the tower and his decision not to call in an artillery strike kept the tower from being destroyed.

On February 27, 1964, the government of Italy requested aid in preventing the tower from toppling. It was, however, considered important to retain the current tilt, due to the vital role that this element played in promoting the tourism industry of Pisa.

A multinational task force of engineers, mathematicians and historians gathered on the Azores islands to discuss stabilisation methods. It was found that the tilt was increasing in combination with the softer foundations on the lower side. Many methods were proposed to stabilise the tower, including the addition of 800 tonnes of lead counterweights to the raised end of the base.

In 1987 the tower was declared as part of the Piazza del Duomo UNESCO World Heritage Site along with the neighbouring cathedral, baptistery and cemetery.

On January 7, 1990, after over two decades of stabilisation studies, the tower was closed to the public. The bells were removed to relieve some weight, and cables were cinched around the third level and anchored several hundred meters away. Apartments and houses in the path of the tower were vacated for safety. The final solution to prevent the collapse of the tower was to slightly straighten the tower to a safer angle, by removing 38 cubic meters (50 cubic yards) of soil from underneath the raised end. The tower was straightened by 45 centimeters (18 inches), returning to its 1838 position. After a decade of corrective reconstruction and stabilization efforts, the tower was reopened to the public on December 15, 2001, and was declared stable for at least another 300 years.

In May 2008, after the removal of another 70 metric tons (77 short tons) of ground, engineers announced that the Tower had been stabilized such that it had stopped moving for the first time in its history. They stated it would be stable for at least 200 years.

Two German churches have challenged the tower's status as the world's most lopsided building: the 15th century square Leaning Tower of Suurhusen and the 14th century bell tower in the town of Bad Frankenhausen. Guinness World Records measured the Pisa and Suurhusen towers, finding the former's tilt to be 3.97 degrees. In June 2010 Guinness World Records certified the Capital Gate building in Abu Dhabi, UAE as the "World's Furthest Leaning Man made Tower". The Capital Gate tower has an 18 degree slope, almost five times more than the Pisa Tower; however the Capital Gate tower has been deliberately engineered to slant. The Leaning Tower of Wanaka in New Zealand, also deliberately built, leans at 53 degrees to the ground.

• Elevation of Piazza del Duomo: about 2 metres (6 feet, DMS)
• Height from the ground floor: 55.863 metres (183 ft 3 in), 8 stories
• Height from the foundation floor: 58.36 metres (191 ft 47 in)
• Outer diameter of base: 15.484 metres (50 ft 9.6 in)
• Inner diameter of base: 7.368 metres (24 ft 2.1 in)
• Angle of slant: 3.97 degrees or 3.9 metres (12 ft 10 in) from the vertical
• Weight: 14,700 metric tons (16,200 short tons)
• Thickness of walls at the base: 2.4 metres (7 ft 10 in)
• Total number of bells: 7, tuned to musical scale, clockwise
  • 1st bell: L'Assunta, cast in 1654 by Giovanni Pietro Orlandi, weight 3,620 kg (7,981 lb)
  • 2nd bell: Il Crocifisso, cast in 1572 by Vincenzo Possenti, weight 2,462 kg (5,428 lb)
  • 3rd bell: San Ranieri, cast in 1719 – 1721 by Giovanni Andrea Moreni, weight 1,448 kg (3,192 lb)
  • 4th bell: La Terza (1st small one), cast in 1473, weight 300 kg (661 lb)
  • 5th bell: La Pasquereccia or La Giustizia, cast in 1262 by Lotteringo, weight 1,014 kg (2,235 lb)
  • 6th bell: Il Vespruccio (2nd small one), cast in the 14th century and again in 1501 by Nicola di Jacopo, weight 1,000 kg (2,205 lb)
  • 7th bell: Dal Pozzo, cast in 1606 and again in 2004, weight 652 kg (1,437 lb)
• Number of steps to the top: 296

About the 5th bell: The name Pasquareccia comes from Easter, because it used to ring on Easter day. However, this bell is older than the bell chamber itself, and comes from the tower Vergata in Palazzo Pretorio in Pisa, where it was called La Giustizia (The Justice). The bell was tolled to announce executions of criminals and traitors, including Count Ugolino in 1289. A new bell was installed in the belltower at the end of the 18th century to replace the broken Pasquareccia.