The procedure for conversion depends on the sponsoring denomination, and depends on meeting the ritual and other requirements for a conversion of that denomination.
It is not necessary for a person to formally convert to Judaism in order to adopt any or all beliefs and practices of Judaism. In Judaism, such people are referred to as righteous gentiles (see, for example the character of Job.) There are various groups that have adopted Jewish customs and practices. For example, in Russia, the Subbotniks have adopted most aspects of Judaism without formal conversion to Judaism
, but there are other groups.
A male convert to Judaism is referred to in Judaism by the biblical word ger (גר, plural gerim) and a female convert is a giyoret. The word corresponds to the term "proselyte", which is derived from the Greek translation known as the Septuagint.
The word "ger" comes from the Hebrew root word gar (גר ) meaning to "live" or "sojourn [with]". In the Hebrew Bible "ger" (and the Septuagint "proselyte") means "stranger" ("newcomer to Israel"; a "sojourner in the land".) Rabbi Marc Angel, a modern Orthodox authority on conversion and former president of the RCA, introduces the history of conversion and the term "ger" as follows:
Angel's explanation of the literal meaning of "ger" as alien is borne out in biblical verses such as :
As Jews were not converts in Egypt, but rather strangers, the verse indicates the meaning of ger as "stranger". There is no place in the Hebrew Bible where the term "ger" is clearly used to refer to a convert to Judaism. The closest thing in the text of the Hebrew Bible to a conversion process is the circumcision undergone by the male stranger ("ger") before eating the Passover offering (). Another relevant passage concerns the non-Jewish woman captured in war. Women captured by the Israelite armies could be adopted forcibly as wives, but first they had to have their heads shaved and undergo a period of mourning.
The Rabbis of the Talmud used the word "ger" to refer to two different types of people: ger tzedek, meaning "righteous convert", a proselyte to Judaism, and ger toshav, a non-Jewish inhabitant of the Land of Israel who observes the Seven Laws of Noah and has repudiated all links with idolatry.. The use of the word ger today corresponds to the persons which the Talmudic called ger tzedek.
However, after a person completes a formal conversion, rabbinic attitude is that he or she should not be referred to as a convert (or ger), but as a citizen, or full member of the Jewish community, and he or she should not be reminded of their earlier lifestyle.
All mainstream forms of Judaism today are open to sincere converts, with all denominations accepting converts converted by their denominations. The rules vary between denominations.
For Rabbinical Judaism, the laws of conversion are based on codes of law and texts, including discussions in the Talmud, through the Shulkhan Arukh, and subsequent interpretations. These rules are held as authoritative by Orthodox Judaism and Conservative Judaism. Jewish law is generally interpreted as discouraging proselytizing, and religious conversion is also discouraged. Rabbis are technically required to reject potential converts three times, and only if they remain adamant to then convert them. This is on two grounds:
However, a rabbi convinced of the prospective convert's sincerity may allow him or her to follow the process of conversion, and thus appear before an established three-judge Jewish religious court known as a beth din ("religious court") to be tested and formally accepted. A person who formally converts to Judaism under the auspices of a halakhically constituted and recognized Beth Din ("Court [of Jewish-Torah Law]"), consisting preferably of three learned rabbis acting as Dayanim ("judges"), but also possibly two learned and respected lay members of the community along with a rabbi, is issued with a Shtar geirut ("Certificate of Conversion").
There is no specific time frame for the conversion process and procedures. In Orthodox Judaism, a prospective convert is taught the basic laws and beliefs of Judaism, and must show an ability to keep the laws and make a commitment to keep them.
Orthodox Judaism recognises only those conversions in which a convert accepts and undertakes to observe halakha as interpreted by the teachings of Orthodox Judaism. Because some non-Orthodox rabbis and some non-Orthodox denominations do not require that converts make this commitment, Orthodox Judaism does not accept those conversions as valid.
Conservative Judaism takes a more lenient approach in application of the halakhic rules than Modern Orthodox Judaism. Its approach to the validity of conversions is based on whether the conversion procedure followed rabbinic norms, rather than the reliability of those performing it or the nature of the obligations the convert undertook. Accordingly, it may accept the validity of some Reform and Reconstructionist conversions, but only if they include immersion in a ritual bath (mikvah), appearance before a rabbinical court (beit din) and, for men, circumcision (brit milah) or a symbolic circumcision for those already circumcised (hatafat dam brit).
The requirements of Reform Judaism for conversions often vary from traditional ones. The denomination states that "people considering conversion are expected to study Jewish theology, rituals, history, culture and customs, and to begin incorporating Jewish practices into their lives. The length and format of the course of study will vary from rabbi to rabbi and community to community, though most now require a course in basic Judaism and individual study with a rabbi, as well as attendance at services and participation in home practice and synagogue life." Reform also note that "Reform, Reconstructionist and under certain circumstances, Conservative rabbis recognize the validity of conversions performed by rabbis of all branches of Judaism. Many Orthodox rabbis, however, do not recognize non-Orthodox conversions".
Although an infant conversion might be accepted in some circumstances (such as in the case of adopted children or children whose parents convert), children who convert would typically be asked if they want to remain Jewish after reaching religious adulthood - which is 12 years of age for a girl and 13 for a boy. This standard is applied by Orthodox and Conservative Judaism, which accept halakha as binding.
Karaite Judaism does not accept Rabbinic Judaism and has different requirements for conversion. Traditionally non-proselytizing, Karaite Judaism's long standing abstention from conversions was recently lifted. On 1 August 2007, the Karaites reportedly converted their first new members in 500 years. At a ceremony in their Northern California synagogue, ten adults and four minors swore fealty to Judaism after completing a year of study. This conversion comes 15 years after the Karaite Council of Sages reversed its centuries-old ban on accepting converts.
However, the Amoraim who produced the Talmud set out three requirements for a conversion to Judaism (Keritot 8b), which must be witnessed and affirmed by a beth din:
The consensus of halakhic authorities also requires a convert to understand and accept the duties of the halakha, classical Jewish religious law. This is not stated explicitly in the Talmud, but was inferred by subsequent commentators. 
After confirming that all these requirements have been met, the beth din issues a "Certificate of Conversion" (Shtar Giur), certifying that the person is now a Jew.
R. Joshua argued that beside accepting Jewish beliefs and laws, a prospective convert to Judaism must undergo immersion in a mikveh (ritual bath, cf. baptism). In contrast, R. Eliezer makes circumcision a condition for the conversion. A similar controversy between the Shammaites and the Hillelites is given (Shabbat 137a) regarding a proselyte born without a foreskin: the former demanding the spilling of a drop of blood of the covenant; the latter declaring it to be unnecessary.
The rigorous view is echoed in the Midrash: "If thy sons accept My Godhead [by undergoing circumcision] I shall be their God and bring them into the land; but if they do not observe My covenant in regard either to circumcision or to the Sabbath, they shall not enter the land of promise" (Midrash Genesis Rabbah xlvi.). "The Sabbath-keepers who are not circumcised are intruders, and deserve punishment," (Midrash Deut. Rabbah i.)
It appears that while the Palestinian Jews accepted the uncircumcised proselytes only as "proselytes of the gate", non-Palestinian Judaism did not make such a distinction until the Jewish-Roman wars, when the more rigorous view became prevalent everywhere. Thus when Flavius Clemens, a nephew of the emperors Titus and Domitian, embraced the Jewish faith with his wife Domitilla, he underwent circumcision and suffered the penalty of death (see Grätz, "Gesch." iv. 403 et seq., 702).
The early Christian community was at that time a sect within Judaism, and took part in the debate. (See Circumcision controversy in early Christianity.) However, the position which the Christian community adopted, without reference to the wider implications, created a divergence of views on the issue. As part of that debate, early Christians advanced concepts such as "circumcision of the spirit" in substitution to "circumcision of the flesh". The debate led Paul of Tarsus to urge the "the seal of baptism" and "circumcision of the heart" in substitution to "circumcision of the flesh" (, and elsewhere), which he considered to be of no particular importance.
Flavius Josephus in Jewish Antiquities book 20, chapter 2 recorded the story of King Izates of Adiabene who decided to follow the Law of Moses at the advice of a Jewish merchant named Ananias. He was going to get circumcised, but his mother, Helen, who herself embraced the Jewish customs, advised against it on the grounds that the subjects wouldn't stand to be ruled by someone who followed such "strange and foreign rites". Ananias likewise advised against it, on the grounds that worship of God was superior to circumcision (Robert Eisenman in James the Brother of Jesus claims that Ananias is Paul of Tarsus who held similar views) and that God would forgive him for fear of his subjects. So Izates decided against it. However, later, "a certain other Jew that came out of Galilee, whose name was Eleazar", who was well versed in the Law, convinced him that he should, on the grounds that it was one thing to read the Law and another thing to practice it, and so he did. Once Helen and Ananias found out, they were struck by great fear of the possible consequences, but as Josephus put it, God looked after Izates. As his reign was peaceful and blessed, Helen visited the Jerusalem Temple to thank God, and since there was a terrible famine at the time, she brought lots of food and aid to the people of Jerusalem.
The requirements for conversion to Judaism are intended to avoid any uncertainty about a convert's true status. The certification by a beth din was based on events the completeness of which were carefully defined.
In the United States of America, Reform Judaism rejects the concept that any rules or rituals should be considered necessary for conversion to Judaism. In the late 1800s, the Central Conference of American Rabbis, the official body of American Reform rabbis, formally resolved to permit the admission of converts "without any initiatory rite, ceremony, or observance whatever." (CCAR Yearbook 3 (1893), 73-95; American Reform Responsa (ARR), no. 68, at 236-237.)
Although this resolution has often been examined critically by many Reform rabbis, the resolution still remains the official policy of American Reform Judaism (CCAR Responsa "Circumcision for an Eight-Year-Old Convert" 5756.13 and Solomon Freehof, Reform Responsa for Our Time, no. 15.) Thus, American Reform Judaism does not require ritual immersion in a mikveh, circumcision, or acceptance of mitzvot as normative. Appearance before a Bet Din is recommended, but is not considered necessary. Converts are asked to commit to religious standards set by the local Reform community. As such, all Reform conversions are rejected as not being conversions by most non-Reform Jews, though under certain circumstances the Conservative Movement accepts Reform conversions (see below).
Various forms of Liberal Judaism in Israel, the United Kingdom and other countries reject this approach. Many Liberal rabbis in these countries hold that it is necessary for a man to have a brit milah or brit dam, that both men and women require immersion in a mikveh, and that the conversion must only be allowed at the end of a formal course of study, before a Bet Din. This is a common view of many Reform rabbis in Canada, as well as a small but growing number of Reform rabbis in the USA.
In suburban areas where there is not a very high Jewish population, cooperation between Modern Orthodox and Conservative rabbis is somewhat more common. Many Conservative rabbis report cooperation in joint conversions with both Modern Orthodox and Conservative rabbis.
In general, branches of Orthodox Judaism consider non-Orthodox conversions either inadequate or of questionable halachic compliance, and such conversions are therefore not accepted by these branches of Judaism. Conversely, both Conservative and Reform Judaism accept the stringent Orthodox conversion process as being valid. Ironically, since 2008, Haredi Orthodox religious courts in Israel have been rejecting conversions from other Orthodox rabbis, in addition to Reform and Conservative conversions, as not being stringent enough.
This issue recently reached a crisis point when the (Orthodox) Chief Rabbinate of Israel changed its requirements for conversion without informing American Orthodox rabbis, and began systematically rejecting most Orthodox Jewish conversions done outside of Israel. The Chief Rabbinate of Israel began to reject all American Orthodox Jewish conversions done by any Orthodox rabbi, except those on a short list of rabbis, numbering less than fifty, some of them deceased. This sparked a public response by Orthodox groups in the United States:
One of the groups promoting this change is the Vaad HaRabbonim Haolami LeInyonei Giyur. They hold that Orthodoxy hasn't had a unified standard of conversion, and that many Orthodox conversions done in the last century are suspect. They criticise as being too lax the Chief Rabbinate of Israel; the Chief Rabbi of the United Kingdom, Jonathan Sacks, and a number of other Orthodox rabbis. Many critiques are aimed at Modern Orthodox Jews, who they label as "so-called Orthodox". Orthodox rabbis who cooperate in any way with non-Orthodox Jews are labeled as "orthodox", in quotes with a lower case "o", while Haredi rabbis are called Orthodox rabbis, with no quotes and no lower case letters. (Eisenstein, About Cooperation with Conservative, Reform -- and So-called Orthodox)
Since 2005, Chief Rabbi Amar of the Israeli rabbinate has been in talks with the Rabbinical Council of America. They tentatively floated a compromise: the RCA would draft a joint list of qualified beth dins authorized to perform conversions in the future, and the Chief Rabbinate of Israel would review the list; all conversions accepted by the RCA in the past and for a short period in the future will be accepted. As of May 2007, this agreement has not been accepted by Rabbi Amar. Rabbi Amar has demanded RCA rabbis on American Beit Din be approved by the Israeli rabbinate. Traditionally, Orthodox communal rabbis have refrained from implementing tests on colleagues from other regions. Rabbi Amar's novel requirement to vet RCA rabbis marks an evolution in diaspora and Israeli rabbinate relations. Rabbi Amar has also demanded more Haredi representation on Israel's conversion courts.
Meanwhile, within Israel, the status of converts is becoming more confused. Recently, an Israeli rabbi in Ashdod retroactively annulled an Orthodox conversion performed by another rabbi whom he regards as not complying with the halakha. 
Recognizing a threat to completed and future conversions, the State of Israel is considering reforming the Haredi dominated Conversion Authority. Under Immigrant Absorption Minister Ze'ev Boim's plan, power would be distributed more widely than at present. According to Erez Halfon, director general of the Absorption Ministry, the current system's "rabbinical courts are intimidating converts as well as rabbis by setting unreasonable requirements." The reform proposes unifying institutions that deal with conversion under one administrative umbrella, and doubling the number of rabbinical judges who review conversions. 
Some Sephardic Orthodox rabbis, particularly those of Judeo-Spanish descent, take a more liberal view of conversion than the majority opinion. For instance, the first Sephardic Chief Rabbi of Israel, Benzion Uziel, held that conversions should be accepted, if not strongly recommended, in the case of an intermarried couple who will continue to live as man and wife regardless of whether the non-Jewish spouse converts. Rabbi Uziel reasoned that it was preferable to convert the non-Jewish partner, regardless of the partner's reasoning for converting, than to permit the Jewish partner to continue transgressing by living with a non-Jew in a spousal relationship. Today, advocates for Rabbi Uziel's opinion include Rabbi Marc D. Angel of Shearith Israel (the Spanish-Portuguese Synagogue) in New York, but very few if any other Orthodox rabbis.
In 2008, the Haredi-dominated supreme Rabbinical Court in Israel annulled thousands of conversions performed by their Modern Orthodox Religious Zionist counterparts in Israel. These converts, ostensibly now unable to marry, be buried in Jewish ceremonies, or have recognized Jewish children, may have to re-convert under Haredi auspices. This controversial ruling is unprecedented in Jewish history.
All conversion candidates – who could include singles, non-Jewish couples and adoption cases – must have a sponsoring rabbi and undergo a rigorous screening process. Conversions stemming from both programs are recognized in Israel and around the world.
For a number of reasons, the project did not succeed. According to Orthodox Rabbi Bernstein, the major reason for its failure was the Orthodox rabbis insisted that the Conservative Rabbinical Assembly expel Conservative rabbis for actions they took before this new beth din was formed, and the RA refused to do so. (Bernstein, 1977) According to Orthodox Rabbi Emanuel Rackman, former president of the RCA, the major reason for its failure was pressure from right-wing Orthodox rabbis, who held that any cooperation between Orthodoxy and Conservatism was forbidden. In 1956, Rabbi Harry Halpen, of the Joint Conference wrote a report on the demise of this beth din. He writes that negotiations between the Orthodox and Conservative were completed and agreed upon, but then a new requirement was demanded by the RCA: The RA must "impose severe sanctions" upon Conservative rabbis for actions they took before this new beth din was formed. Halpern writes that the RA "could not assent to rigorously disciplining our members at the behest of an outside group." He goes on to write that although subsequent efforts were made to cooperate with the Orthodox, a letter from eleven Rosh Yeshivas was circulated declaring that Orthodox rabbis are forbidden to cooperate with Conservative rabbis. (Proceedings of the CJLS of the Conservative Movement 1927-1970 Vol. II, p.850-852.)
The end of this program was welcomed by Haredi Orthodox groups, who saw the program as illegitimate. Further, Haredi groups attempted to prevent non-Orthodox rabbis from following the traditional requirements of converts using a mikvah. In the Haredi view, it is better to have no conversion at all than a non-Orthodox conversion, as all non-Orthodox conversions are not true conversions at all according to them. (Fifth Anniversary of the Mikveh of East Denver, Hillel Goldberg)
A plan was developed by Israeli Cabinet Secretary Elyakim Rubenstein, who negotiated secretly for many months with rabbis from Conservative, Reform and Orthodox Judaism, including faculty at Yeshiva University, with Lamm as Rosh Yeshiva. They were planning to create a joint panel that interviewed people who were converting to Judaism and considering making aliyah (moving to the State of Israel), and would refer them to a beth din that would convert the candidate following traditional halakha. All negotiating parties came to agreement:
Many Reform rabbis took offense at the notion that the beth din must be strictly halakhic and Orthodox, but they acquiesced. However, when word about this project became public, a number of leading haredi rabbis issued a statement denouncing the project, condemning it as a "travesty of halakha. Rabbi Moshe Sherer, Chairman of Agudath Israel World Organization, stated that "Yes we played a role in putting an end to that farce, and I'm proud we did." Norman Lamm condemned this interference by Sherer, stating that this was "the most damaging thing that he [Sherer] ever did in his forty year career." (Landau, p.320)
Rabbi Lamm wanted this to be only the beginning of a solution to Jewish disunity. He stated that had this unified conversion plan not been destroyed, he wanted to extend this program to the area of halakhic Jewish divorces, thus ending the problem of mamzerut. (Landau, p.320)
The plan has been effectively rendered non-existent due to denunciations from haredi rabbis, causing some other Orthodox rabbis to back out, and causing the Israeli Chief rabbinate to not support this program.
A public and clear violation of Jewish law immediately following the formal conversion may give grounds for considering such an annulment if it can be demonstrated that such a violation proved that the conversion was fraudulent to begin with. A few isolated cases of annulments have been vigorously debated within the Jewish community, with several rabbinical authorities holding the practice to be in violation of halakha
According to Orthodox interpretations of Halakha, converts face a limited number of restrictions. A marriage between a female convert and a kohen (members of the priestly class) is prohibited and any children of the union do not inherit their father's kohen status. Converts can become rabbis. For instance, Rabbi Meir Baal Ha Nes is thought to be a descendant of a proselyte. Indeed the Talmud lists many of the Jewish nation's greatest leaders, who had either descended from or were themselves converts. In fact, King David is descended from Ruth, a convert to Judaism. While a Jew by birth may not marry a mamzer a convert can marry a mamzer.
In Orthodox and Conservative communities which maintain tribal distinctions, converts become Yisraelim, ordinary Jews with no tribal or inter-Jewish distinctions. They traditionally become part of the tribe of Judah. Converts typically follow the customs of their congregations. So a convert who prays at an Sephardi synagogue would follow Sephardi customs and learn Sephardi Hebrew.
A convert chooses his or her own Hebrew first name upon conversion but is traditionally known as the son or daughter of Abraham and Sarah, the first patriarch and matriarch in the Torah, often with the additional qualifier of "Avinu" (our father) and "Imenu" (our mother). Hence, a convert named Akiva would be known, for ritual purposes in a synagogue, as "Akiva ben Avraham Avinu"; in cases where the mother's name is used, such as for the prayer for recovery from an illness, he would be known as "Akiva ben Sarah Imenu".
Talmudic opinions on converts are numerous; some positive, some negative. A famous quote from the Talmud, labels the convert "Hard on Israel as a blight." Many interpretations explain this quote as meaning converts can be unobservant and leading Jews to be unobservant, or converts can be so observant that born Jews feel ashamed.
For purely rhetorical purposes, some polemicists elicit that every Jew is a Jew by choice, because the worldwide Jewish community is so small and the pull of assimilation is so great. So it is very easy for someone who was born Jewish to abandon Jewish traditions and customs in adulthood, absent a conscious choice to stay Jewish. This perspective is not part of Jewish law or social custom.
For purely philosophical purposes, a convert is not considered by many to be a "Jew by Choice" at all. In fact, many traditional Jews take offense not only at the word "convert" but also at this more politically-correct term. The reasoning for this is the notion that everyone has a Jewish soul, and that those who were not born Jewish, and elect to go through the conversion process, have returned to the nature of their soul through true teshuva. Therefore, this person is not to be known as a convert, or any derivation of the term.