The Leaning Tower of Pisa or simply the Tower of Pisa is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa, known worldwide for its unintended tilt. The tower is situated behind the Pisa Cathedral and is the third oldest structure in the city's Cathedral Square (Piazza del Duomo), after the cathedral and the Pisa Baptistry.

The tower's tilt began during construction in the 12th century, caused by an inadequate foundation on ground too soft on one side to properly support the structure's weight. The tilt increased in the decades before the structure was completed in the 14th century. It gradually increased until the structure was stabilized (and the tilt partially corrected) by efforts in the late 20th and early 21st centuries.

The height of the tower is 183.27 feet from the ground on the low side and 185.93 feet on the high side. About 8 stories from the ground floor.  The width of the walls at the base is 8 ft 0.06 in. Its weight is estimated at 14,500 metric tons. The tower has 296 or 294 steps; the seventh floor has two fewer steps on the north-facing staircase. In 1990 the tower leaned at an angle of 5.5 degrees, but following remedial work between 1993 and 2001 this was reduced to 3.97 degrees, reducing the overhang by 45 cm at a cost of Ł200m. It lost a further 4 cm of tilt in the two decades to 2018.

Construction of the tower occurred in three stages over 199 years. Work on the ground floor of the white marble campanile began on August 14, 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-meter 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 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 curved. Construction was halted again in 1284 when the Pisans were defeated by the Genoans in the Battle of Meloria.

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

Galileo Galilei is said to have dropped two cannonballs of different masses from the tower to demonstrate that their speed of descent was independent of their mass. However, the only primary source for this is the biography Historical Account of the Life of Galileo Galilei, written by Galileo's secretary Vincenzo Viviani and published in 1717, long after Viviani's death.

During World War II, the Allies suspected that the Germans were using the tower as an observation post. A U.S. Army sergeant sent to confirm the presence of German troops in the tower was impressed by the beauty of the cathedral and its campanile, and thus refrained from ordering an artillery strike, sparing it from destruction.

Numerous efforts have been made to restore the tower to a vertical orientation or at least keep it from falling over. Most of these efforts failed; some worsened the tilt. 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 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 stabilization methods. It was found that the tilt was increasing in combination with the softer foundations on the lower side. Many methods were proposed to stabilize the tower, including the addition of 800 tonnes of lead counterweights to the raised end of the base.

The tower and the neighboring cathedral, baptistery, and cemetery are included in the Piazza del Duomo UNESCO World Heritage Site, which was declared in 1987.

The tower was closed to the public on January 7, 1990, after more than two decades of stabilization studies and spurred by the abrupt collapse of the Civic Tower of Pavia in 1989. 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 solution chosen to prevent the collapse of the tower was to slightly straighten it to a safer angle by removing 1,342 cubic feet of soil from underneath the raised end. The tower was straightened by 17.7 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 total, 70 metric tons (77 short tons) of earth were removed.

In May 2008, engineers announced that the tower had been stabilized such that it had stopped moving for the first time in its history. They stated that it would be stable for at least 200 years.

At least four strong earthquakes hit the region since 1280, but the apparently vulnerable Tower survived. The reason was not understood until a research group of 16 engineers investigated. The researchers concluded that the Tower was able to withstand the tremors because of dynamic soil-structure interaction (DSSI): the height and stiffness of the Tower together with the softness of the foundation soil influences the vibrational characteristics of the structure in such a way that the Tower does not resonate with earthquake ground motion. The same soft soil that caused the leaning, and brought the Tower to the verge of collapse helped it survive.