Judaism
Jews circumcise their newborn boys as a physical sign of God's covenant with the Jewish people. The circumcision ceremony marks a boy's welcome into this covenant and into the Jewish community.
Brit milah, literally, Covenant of Circumcision.
Circumcision is performed on the child's eighth day of life, including the day of the birth.
The mohel, a religious Jew educated in Jewish law and in the circumcision ritual.
"And G-d said unto Abraham: 'And as for thee, thou shalt keep My covenant, thou, and thy seed after thee throughout their generations. This is My covenant, which ye shall keep, between Me and you and thy seed after thee: every male among you shall be circumcised. And ye shall be circumcised in the flesh of your foreskin; and it shall be a token of a covenant betwixt Me and you. And he that is eight days old shall be circumcised among you, every male throughout your generations, he that is born in the house, or bought with money of any foreigner, that is not of thy seed...and My covenant shall be in your flesh for an everlasting covenant. And the uncircumcised male who is not circumcised in the flesh of his foreskin, that soul shall be cut off from his people; he hath broken My covenant.'"
--Genesis 17:9-14
An anti-circumcision movement has mounted in recent years, with active and vocal Jewish members. Anti-circumcision groups claim the act is traumatic for the baby and can result in lifelong damage, including psychological problems and a reduced sex drive. For more on this topic, see:
A Jewish mother looks into the reasons for performing the brit milah ceremony.
Additional Jewish circumcision resources:
Circumcision is not symbolic of being part of the Muslim community, as it is in Judaism, but most Muslims circumcise their sons. Though circumcision is not specifically mandated or mentioned in the Qur'an, many Muslims believe it is necessary because Allah ordered Muhammad to follow the way of Abraham, who circumcised himself. Many Muslims also believe circumcision is a form of cleanliness.
Muslim boys can be circumcised on the seventh day of life, but the ritual can be done any time during the first few years of life, as long as it is before a boy's seventh year.
"Allah's Apostle said, 'Five practices are characteristics of the Fitra: circumcision, shaving the pubic region, clipping the nails and cutting the moustaches short.'"
--Hadith narrated by Abu Hurairah
Islam is often wrongly accused of condoning female genital mutilation, or 'female circumcision,' the cutting or removal of a girl's clitoris. In fact, female genital mutilation was practiced long before the advent of Islam, and the custom likely has ancient roots. Several hadith (sayings of the Prophet) do refer to female circumcision, but it is not practiced by most Muslims. For more on this controversy, see:
Additional Muslim circumcision resources:
Other Religions
Most Christians do not circumcise their male children, though in the United States many babies are circumcised in the hospital for health reasons. Christians do not believe it is a religious obligation to circumcise because Christians are not obligated to keep the laws of the Old Testament. In Romans, Paul wrote that "real circumcision is a matter of the heart" (Romans 2:29).
See also:
Why Christians Don't Circumcise
By Thomas Lynch
Circumcision is often practiced among Coptic (Egyptian) Christians.
Buddhists, Hindus, and Pagans do not practice circumcision, though they do have numerous welcoming and other rituals for babies.