As important was the sanctuary of Apollo at Delphi, so was the big sanctuary-oracle of Didymaios Apollo in the region of Ionia. In its final form, the complex was one of the three largest sanctuaries-oracles of the Greek world at the time.
The sanctuary and the location of Didyma were accessible by the sea and by a processional way starting from Miletus, which was also the city-protector of the oracle of Didyma.
The foundation of the sanctuary is dated in the period after the Ionian settlement in the area and not before 900 B.C. The oracle and the sanctuary are for the first time referred in sources of the 6th B.C. Before the above period, the complex comprised only of the altar and a spring, which were enclosed inside the 6th century archaic dipteral temple. After 494 B.C., when the Ionian Revolt against the Persians was suppressed, the sanctuary was looted and destroyed. It re-worked in the next century, and according to sources, in 331 B.C. one of the first prophecies was given to Alexander the Great. A new construction phase began in 300 B.C. with the support of Seleucus I. This costly project lasted almost five centuries and even in the 2nd century A.D. the works continued in the temple, while many of its parts were never completed.
The sanctuary kept on working in the 3rd and 4th century A.D. till the period 390-400, when the Christians of Miletus transformed parts of the temple into a basilica. The monument passed to the Seljuk Turks and to the Ottomans at the beginning of the 14th century. Later, it was abandoned.
The systematic excavation research in the sanctuary-oracle was performed by the German Archaeological Institute after the Second World War.