The Serapeum or Sarapeion, which contained the Temple of Serapis, was Alexandria's most important sanctuary, and one of the most famous pagan sanctuaries of antiquity. It was also the centre of a cult which spread widely across the Mediterranean in the Hellenistic and Roman periods. Although the excavation of the Serapeum was completed half a century ago, the archaeological evidence for its form and phases has never been fully collected and analysed. When the records of the remains uncovered in c. 1900 are combined with those of the excavations during World War II, analysis of them reveals that there is sufficient evidence from which to suggest reliable reconstructions of both the Ptolemaic and Roman phases of the complex and to clarify its chronology. The archaeological evidence also elucidates the information in the written sources about the conversion of the site to Christian use after the destruction of the temple in A.D. 391. Previously unpublished architectural fragments excavated at the site in c. 1900 suggest that the architecture of the Ptolemaic sanctuary was ‘classical’ (Greek) not Egyptian in style (see Appendix).