Summary: The view of the Egyptological mainstream, according to which the Great Pyramid of Giza has been built with limestone, quarried in its immediate neighbourhood, has to be judged as untenable on more than one account. It seems to have been built mainly with blocks of calcareous sandstone, quarried in other parts of Egypt. Only a sufficiently comprehensive series of technical tests on stone blocks from the Great Pyramid, as well as on natural rock from as many ancient quarries in Egypt as possible, can clear up this up to now strangely neglected problem. The white limestone mantle of the pyramid seems to belong to the realm of speculation. In view of the technical and logistic problems involved in a project of such magnitude, the question seems legitimate if the ancient Egyptians were able to shoulder these alone, or if they were assisted by specialists from some as yet unidentified, more advanced civilization.