Finding the ten lost tribes of Israel, constituting the northern Kingdom of Israel. The Kingdom of Israel, consisting of the ten tribes (the twelve tribes excluding Judah and Benjamin who constituted the southern Kingdom of Judah), which fell in 722 B.C.E. and its inhabitants were exiled to "Halah and Habor by the river Gozan, and in the cities of the Medes" (II Kings 17:6 and 18:11). However, the parallel passage in I Chronicles 5:26 to the effect that the ten tribes were there "unto this day" and the prophecies of Isaiah (11:11), Jeremiah (31:8), and above all of Ezekiel (37: 19–24) kept alive the belief that they had maintained a separate existence and that the time would come when they would be rejoined with their brethren, the descendants of the Exile of Judah to Babylon. Their place in history, however, is substituted by legend, and the legend of the Ten Lost Tribes is one of the most fascinating and persistent in Judaism and beyond it.

The belief in the continued existence of the ten tribes was regarded as an incontrovertible fact during the whole period of the Second Temple and of the Talmud. Josephus (Ant., 11:133) states as a fact "the ten tribes are beyond the Euphrates till now, and are an immense multitude and not to be estimated in numbers." The only opposing voice to this otherwise universal view is found in the Mishnah. R. Eliezer expresses his view that they will eventually return and "after darkness is fallen upon the ten tribes light shall thereafter dwell upon them," but R. Akiva expresses his emphatic view that "the ten tribes shall not return again" (Sanh. 10:3). In consonance with this view, though it is agreed that Leviticus 26:38 applies to the ten tribes, where R. Meir maintains that it merely refers to their exile, R. Akiva states that it refers to their complete disappearance (Sifra, Be-Hukkotai, 8:1).

Their inability to rejoin their brethren was attributed to the fact that whereas the tribes of Judah and Benjamin (the Kingdom of Judah) were "scattered throughout the world," the ten tribes were exiled beyond the mysterious river Sambatyon (Gen. R. 73:6), with its rolling waters or sand and rocks, which during the six days of the week prevented them from crossing it, and though it rested on the Sabbath, the laws of the Sabbath rendered the crossing equally impossible. According to the Jerusalem Talmud, however (Sanh. 10:6, 29c), the exiles were divided into three. Only one-third went beyond the Sambatyon, a second to "Daphne of Antioch," and over the third "there descended a cloud which covered them"; but all three would eventually return.

Throughout the Middle Ages and until comparatively recent times there were claims of the existence of the ten lost tribes as well as attempts by travelers and explorers, both Jewish and non-Jewish, and by many naive scholars, both to discover the ten lost tribes or to identify different peoples with them. In the ninth century Eldad ha-Dani claimed not only to be a member of the tribe of Dan, but that he had communicated with four of the tribes. David Reuveni claimed to be the brother of Joseph the king of the tribes of Reuben, Gad, and the half-tribe of Manasseh who were settled in Khaybar in Arabia, which was identified with the Habor of II Kings. Benjamin of Tudela has a long description of the ten tribes. According to him the Jews of Persia stated that in the town of Nishapur dwelt the four tribes of Dan, Asher, Zebulun, and Naphtali, who were then governed "by their own prince Joseph Amarkala the Levite [ed. by N.M. Adler (1907), 83], while the Jews of Khaybar are of the tribes of Reuben and Gad and the half-tribe of Manasseh" (ibid., 72), as was also stated by Reuveni. Persistent was the legend that they warred with Prester John in Ethiopia, a story repeated by Obadiah of Bertinoro in his first two letters from Jerusalem in 1488 and 1489. The kabbalist Abraham Levi the elder, in 1528, identified them with the Falashas. R. Abraham Farissol gives a long account of them based upon conversations with David Reuveni not to be found in the latter's diary, while the most expansive is that of R. Abraham Jagel, an Italian Jew of the 16th–17th centuries, in the 22nd chapter of his Beit Ya'ar ha-Levanon.