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.

Jewish religious movements, sometimes called "denominations", include diverse groups within Judaism which have developed among Jews from ancient times. Today in the west, the most prominent divisions are between traditionalist Orthodox movements and modernist movements such as Reform Judaism originating in late 18th century Europe, Conservative originating in 19th century Europe, and other smaller ones, including the Reconstructionst and Renewal movements which emerged later in the 20th century in the United States.