Written some time before 100 BC the On the Erythraean Sea of Agatharchides of Cnidus is the most important source for an almost forgotten chapter of the history of geographical discovery, the exploration of the Red Sea and the region surrounding it by agents of the Ptolemaic government of Egypt in the century after the death of Alexander the Great in 323 BC. It also contains the earliest extensive account of the geography and ethnography of the coasts of northeast Africa and western Arabia. Although the original text of the On the Erythraean Sea is no longer extant, three abridgements of unequal extent and quality by the first century BC historian Diodorus, the first century AD geographer Strabo, and the ninth century AD Patriarch of Constantinople Photius survive. The present edition contains the first English translation of all three epitomes of this important work together with an introduction and extensive notes analysing the historical background and significance of Agatharchides' book.