One of the main tourist attractions in Istanbul, the Hagia Sophia known as Ayasofya in Turkish, is also referred to as the Church of St Sophia. It is the third building on the site which started as a church. Initially built on the orders of the Byzantine Emperor Constantine II in 360 AD it was destroyed in 404 AD by a mob and was rebuilt to grander proportions by Theodosius II in 405 AD only to destroyed again in 532 during the week of riots known as the 'Nika Revolt', something which resulted in almost half of Istanbul being burnt down. Rebuilt under Emperor Justinian who was to construct a larger and more majestic building than its predecessors, which resulted in him shipping marble columns to Constantinople for the project from Baalbek in the Lebanon, when completed it was to remain one of the foremost churches despite being damaged by a series of earthquakes in 553, 557 and again in 558. The latter of which resulted in the collapse of the main dome due to it being too flat to transmit the loading down through the piers. The new dome was to survive until a devastating earthquake in 989 AD caused the whole building to crumble, although it was rebuilt to its former glory.

In 1453, Constantinople was conquered by the Ottoman Turks under Sultan Mehmed II, who ordered the Hagia Sophia converted from a church into a mosque. This resulted in the removal of the alter, its bells and iconostasis and the plastering over of the Christian mosaics, which were replaced with geometrical designs. The Ottomans made extensive use of expensive coloured stones, carved wood, gold, mother of pearl and precious stones in this conversion. The 'mihrab' (semicircular niche in the wall of a mosque that indicates the direction of Mecca) (this was located in the apse where the altar used to stand); 'minbar' (pulpit in the mosque where the imam stands to deliver sermons); and the four 'minarets' outside were subsequently added during the period of Ottoman rule. It was to remain an Islamic mosque till 1935, when it was transformed into a museum.

Remnants of the previous churches can be seen located within the grounds; these include the marble blocks from the second church. One showing 12 lambs, which represent the 12 apostles, these were originally part of a front entrance and can be seen adjacent to the museum's entrance. Also to be seen outside is the fountain for ritual ablutions. These are found outside all mosques for the faithful to perform their ablutions before entering. The minarets were constructed over a period of time by different Sultans and whereas three are constructed of white limestone the forth was built from red brick.

On entry, the Imperial Gate was the main entrance to the church and was reserved for the emperor. The Byzantine mosaic above the portal dates to the late 9th or early 10th century, and depicts Christ and Emperor Leo VI. Located above the southwestern entrance and dating from 944 is the mosaic of the Virgin Mary sitting on a backless throne with her feet resting on a pedestal with the child Christ sitting on her lap. To her left side stands Emperor Constantine who is presenting her with a model of the city while on her right is the Emperor Justinian I offering her a model of the Hagia Sophia.

The Hagia Sophia contains a dome which reaches a height of 55.6 metres. It has a diameter of 31.24 metres and is supported by a series of 40 arched windows which provide light to the building. These are held in place by four concave triangular pillars or pendentives (a constructive technique permitting the placing of a circular dome over a square room or an elliptical dome over a rectangular room) which were covered with marble and mosaics, although in later years these needed to be reinforced by the use of buttresses. The dome is shaped like the inside of an umbrella with ribs running from the top down to its base which direct the loading between the windows and down through the pillars to the foundations.

It also contains many columns, the largest of which are of granite, 20 metres in height and in excess of 1.5 metres in diameter; the largest weighs over 70 tons.The walls, ceilings and columns are covered with inlayed marble and mosaics. A ramp leads to the upper galleries that overlook the nave. The central part of the Upper Imperial gallery was, during Byzantine rule, reserved for the Empress and the ladies of her court and was known as the Loge of the Empress. It was there that the Empress would sit and watch the proceedings; the place where the throne of the Empress stood is marked by a round, green stone. Other parts of the galleries contained meeting rooms and in later years a library. It is the galleries which contain some of the finest mosaics of the Hagia Sophia including that of Christ in a blue robe flanked by Empress Zoe and Constantine IX which dates from the 11th Century; Mary with the Child Christ in her arms flanked by Emperor John II and Empress Eirene; and the representation of Judgment day with Christ in the centre, with the Virgin Mary and John the Baptist on either side. Also to be seen are the two large marble lustration (Purification) urns dating from the Hellenistic period (323 BC to about 146 BC), which were carved from single blocks of marble.

Restoration work began in and continued throughout the 20th century and is still in progress today. The difficulty is to obtain a balance between uncovering the ancient mosaics while also preserving the Islamic work.

The Hagia Sophia is one of the greatest surviving examples of Byzantine architecture and was, until the completion of the Seville Cathedral in the 16th century, the largest cathedral for 1000 years. For almost 500 years the Hagia Sophia was the principal mosque of Istanbul, and served as a model for many other Ottoman mosques, including the Sultan Ahmed Mosque or Blue Mosque