Conversion to Judaism is the process by which non-Jews adopt the Jewish religion and become members of the Jewish ethnoreligious community. It thus resembles both conversion to other religions and naturalization. The procedure and requirements for conversion depend on the sponsoring denomination. Furthermore, a conversion done in accordance with one Jewish denomination is not a guarantee of recognition by another denomination. Normally, though not always, the conversions performed by more stringent denominations are recognized by less stringent ones, but not the other way around. A formal conversion is also sometimes undertaken by individuals whose Jewish ancestry is questioned or uncertain, even if they were raised Jewish, but may not actually be considered Jews according to traditional Jewish law.

Judaism is an Abrahamic, monotheistic, and ethnic religion, comprises the collective spiritual, cultural, and legal traditions of the Jewish people. Contemporary Judaism having originated as an organized religion in the Middle East during the Bronze Age, and evolved from Yahwism, the cultic polytheistic religious movement of ancient Israel and Judah, around the 6th/5th century BCE, and is thus considered to be one of the oldest monotheistic religions. Along with Samaritanism, to which it is closely related, Judaism is one of the two oldest Abrahamic religions.