[1] It greatly expanded under the Old and Middle Kingdoms but is today mostly destroyed, its temples and other buildings having been scavenged for the construction of medieval Cairo.

Helios, the personified and deified form of the sun, was identified by the Greeks with the native Egyptian gods Ra and Atum, whose principal cult was located in the city.

Its native name was iwnw ("The Pillars"), whose exact pronunciation is uncertain because ancient Egyptian recorded only consonantal values.

A decline in the importance of Ra's cult during Dynasty V led to the development of the Ennead, a grouping of nine major Egyptian deities that placed the others in subordinate status to Ra–Atum.

[13] During the Amarna Period of Dynasty XVIII, Pharaoh Akhenaten introduced a kind of henotheistic worship of Aten, the deified solar disc.

[16] The temple of Ra was said to have been, to a special degree, a depository for royal records, and Herodotus states that the priests of Heliopolis were the best informed in matters of history of all the Egyptians.

Ptolemy II had Manetho, the chief priest of Heliopolis, collect his history of the ancient kings of Egypt from its archives.

The later Greek rulers, the Ptolemies, probably took little interest in their "father" Ra as Greeks were never much of sun worshipers and the Ptolemies favored the cult of Serapis, and Alexandria had eclipsed the learning of Heliopolis; thus with the withdrawal of royal favour Heliopolis quickly dwindled, and the students of native lore deserted it for other temples supported by a wealthy population of pious citizens.

During the Middle Ages, the growth of Fustat and Cairo only a few kilometres away caused its ruins to be massively scavenged for building materials, including for their city walls.

Classical mythology held that the Egyptian bennu, renamed phoenix, brought the remains of its predecessor to the altar of the sun god at Heliopolis each time it was reborn.

The ancient city is currently located about 15–20 meters (49–66 ft) below the streets of the middle- and lower-class suburbs of Al-Matariyyah,[1] Ain Shams, and Tel Al-Hisn[24] in northern Cairo.

[1] Some ancient city walls of crude brick can be seen in the fields, a few granite blocks bearing the name of Ramesses II remain, and the position of the great Temple of Ra-Atum is marked by the Al-Masalla obelisk.