Who built the Great Mosque of Isfahan?

1. al-Mu'tasim (794-842), the Arab Abbasid Caliph (son of Harun ar-Rashid)

2. Adud ad-Dawlah (936-983), the Persian Buyid King

3. Malik Shah (1055-1092), the Turkish Seljuk Sultan

4. Oljeitu (1280-1316), the Mongol Ilkhanid Emperor

5. Shahrokh (1377-1477), the Turkic Timurid Emperor (son of Timur [Tamerlane])

6. Uzun Hasan (1423-1478), the Turkmen Ak Koyunlu King

7. Shah Suleiman (r. 1667-1694), the Safavid King

The project as an Islmaic wonder was strated by al-Mu'tasim around 837 AD, it is said the step by step process of building this mosque was designed by great mathematician Algorithm.

The mosque has eight entrance gates, the oldest one on Hatef street is a manifestation of brilliant craftsmanship. The small platforms on the right - hand side of the entrance corridor belong to Buyid era the 11th Century.

Southern Portico (Eivan)

This portico leads into some of the oldest parts of the mosque which were built in the Abbasid period.

The foundations for this porch were laid during the Seljuk period at the start of the 12th Century by Nizam Al Mulk, although there was certainly an older building of some kind here as evidenced by the walls which have been found below the current floor of the interior and which were being excavated in April 1999. The roof was rebuilt by Uzun Hasan in 1475/6 according to an inscription in the interior and the two minarets were added at the same time. The marble plinths and some of the raised mosaic tilework inside the eivan date from Uzun Hasan's time. Further extensive modifications took place under the Safvid king Shah Tahmasb.

Shabestan

On the northern side of the great Western Eivan of the mosque there is an impressive portal which leads to the winter Prayer Gallery. The portal is of somewhat later date than the buildings behind it which probably date from 1310 according to the date on the magnificent stucco mihrab of Oljeitu which forms the central feature, and dates from 1447. It was built by Imad ibn Muzaffar Varzaneh, during the reign of Sultan Muhammad Bahadur, one of Shahrokh's grandchildren, possibly as an act of piety following his grandfather's death in 1447. The inscription in mosaic faience above the door reads as follows:

The least of slaves, Imad ibn Muzaffar Varzaneh ordered the construction of this broad structure to be the winter mosque of this noble jami' during the reign of Sultan Muhammad Bahadur ... Shavval 851. Written by Seyyed Mahmud Naqqash

Seljuk Entrance

As you enter the mosque by the South Eastern entrance you are plunged into a series of cloisters which are punctuated by open and closed vaults giving strong contrasts between light and dark and imposing a need for spatial and ocular re-adjustment. Here we find a magnificent example of the "mystical mathematics of the City of Heaven" of which Sir Thomas Browne wrote.
The mystical mathematics were well known to the Sufi craftsmen who designed this part of the mosque. Their system of mathematics was called abjad, and provided a conceptual springboard from which the diverse and inventive geometrical patterns in the roof vaults were derived. Originally it was probably a huge library, and this hypothesis is borne out by the presence of lampholders in part of the cloisters, but after the siege of Isfahan by Toghril Begh, during which large parts of the mosque were ransacked to provide winter fuel, it was rebuilt in its present form.
It was amongst these strange cloisters that one of the greatest Iranians of all time, Nizam al-Mulk, the vizier of Malik Shah and friend and patron of Omar Khayyam, chose to erect his prayer chamber.

Eastern Portico (Eivan)

The complex vaulting of the 12th century Eastern eivan, was added by Shah Suleiman in 1689 as indicated by the tiled inscription.

Winter Prayer Gallery

There is some confusion as to whether this gallery is the winter gallery referred to in the inscription of the doorway. The low sprung vaults which are said to be based on the tents of the mongols are unparalleled anywhere else in Timurid architecture. Each of the eighteen vaults which make up the gallery was formerly lit through an alabaster panel let into the centre of the arch, although this faint illumination has now been replaced by chandeliers and central heating has also been put in.

The wonderful mihrab of Oljeitu:

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The great Seljuk brickwork:

This is more than 2 meters high:

From various books I have on Islamic/Seljuk architecture, they agree to many parts of the construction, an example is that of the 'main dome' that it was started to be built in 1080 by the orders of Malikshah; continueing with a quote from the book.

...Dome is supported by compound piers made of brick,by displaying a unity from the base to the key stone, by its trefoil squinches and slightly pointed form with its overall effect. The substructure srated on the mihrab wal and on a group of eight piers on three sides having three openings each,enlivens when it reaches the squinches and carry the dome. Thus space assambled under a single dome in front of the mihrab could be linked to the subsidiary parts having a lower covering system. Thus the connection to the courtyard with a wide iwan enables a powerfull link of the interior space with the courtyard but retains consistency in the architechtual environment. The same system is employed in the dome of the Terken Khatun on the otherside of the mosque. Seljuk mosque's of this type where piers are joined to the dome are generally called of 'Kiosk Type' mosque, these mosques formed by iwan-dome union, always having lower sections besides them were, in a very short time enlarged by the addition of 2 or 4 iwan courtyards and lateral parts...best example to this is Masijd i Juma of Isfahan

..I also remmember reading somewhere that the structure is stated around something older..but I might be wrong, as I've seen too much architecture of the Seljuks lately.   
   Im working on the wildturks site, when I come to the part on civilian structures of the Seljuks related to this subject, I'll post it here also..