R' Aryeh Kaplan in Waters of Life connects the laws of impurity to the narrative in the beginning of Genesis. According to Genesis, By eating of the fruit Adam and Eve had brought death into the world. Kaplan points out that most of the laws of impurity relate to some form of death (or in the case of Niddah the loss of a potential life). One who comes into contact with one of the forms of death must then immerse in water which is described in Genesis as flowing out of the Garden of Eden (the source of life) in order to cleanse oneself of this contact with death (and by extension of sin).

Orthodox Judaism

Orthodox Judaism generally adheres to the classical regulations and traditions, and consequently Orthodox Jewish women are obligated to immerse in a mikveh between Niddah and sexual relations with their husbands. This includes brides before their marriage, and married women after their menstruation period or childbirth. In accordance with Orthodox rules concerning modesty, men and women are required to immerse in separate mikveh facilities in separate locations, or to use the mikveh at different designated times.

Converts to Orthodox Judaism, regardless of gender, are also required to immerse in a mikveh. It is customary for Orthodox Jews to immerse before Yom Kippur,  and married women sometimes do so as well. In the customs of certain Jewish communities, men also use a mikveh before Jewish holidays;  the men in certain communities, especially hasidic and haredi groups, also practice immersion before each Shabbat, and some immerse in a mikveh every single day. Although the Temple Mount is treated by many Orthodox Jewish authorities as being forbidden territory, a small number of groups permit access, but require immersion before ascending the Mount as a precaution.

Orthodox Judaism requires that vessels and utensils must be immersed in a mikveh before being used for food, if they had been purchased from a non-Jew.

Obligatory immersion in Orthodox Judaism

Immersion in a mikveh is obligatory in contemporary Orthodox Jewish practice in the following circumstances:

• Women
  • Following the niddah period after menstruation, prior to resuming marital relations
  • Following the niddah period after childbirth, prior to resuming marital relations
  • By a bride, before her wedding
• Either gender.
  • As part of a conversion to Judaism
• Immersion of utensils acquired from a gentile

Customary immersion in Orthodox Judaism

Immersion in a mikveh is customary in contemporary Orthodox Jewish practice in the following circumstances:

• Men
  • By a bridegroom, on the day of his wedding, according to the custom of some communities
  • By a father, prior to the circumcision of his son, according to the custom of some communities[which?].
  • By a kohen prior to a service in which he will recite the priestly blessing, according to the custom of some communities
  • Before Yom Kippur, according to the custom of some communities
  • Before a Jewish holiday, according to the custom of some communities
  • Weekly before Shabbat, under Hasidic and Haredi customs
  • Every day, under Hasidic customs

Immersion for men is more common in Hasidic communities, and non-existent in others, like Yekke communities.

Conservative Judaism

In a series of responsa on the subject of Niddah in December 2006, the Committee on Jewish Law and Standards of Conservative Judaism reaffirmed a requirement that Conservative women use a mikveh monthly following the end of the niddah period following menstruation, while adapting certain leniencies including reducing the length of the period. The three responsa adapted permit a range of approaches from an opinion reaffirming the traditional ritual to an opinion declaring the concept of ritual purity does not apply outside the Temple in Jerusalem, proposing a new theological basis for the ritual, adapting new terminology including renaming the observances related to menstruation from taharat hamishpacha family purity to kedushat hamishpaha [family holiness] to reflect the view that the concept of ritual purity is no longer considered applicable, and adopting certain leniencies including reducing the length of the niddah period.

Isaac Klein's A Guide to Jewish Religious Practice, a comprehensive guide frequently used within Conservative Judaism also addresses Conservative views on other uses of a mikveh, but because it predates the 2006 opinions it describes an approach more closely resembling the Orthodox one and does not address the leniencies and views those opinions reflected. Rabbi Miriam Berkowitz's recent book Taking the Plunge: A Practical and Spiritual Guide to the Mikveh (Jerusalem: Schechter Institute, 2007) offers a comprehensive discussion of contemporary issues and new mikveh uses along with traditional reasons for observance, details of how to prepare and what to expect, and how the laws developed. Conservative Judaism encourages but does not require immersion before Jewish Holidays (including Yom Kippur), nor the immersion of utensils purchased from non-Jews. New uses are being developed throughout the liberal world for healing (after rape, incest, divorce etc.) or celebration (milestone birthdays, anniversaries, ordination, or reading Torah for the first time).

Although monthly immersion is formally required of Conservative Jewish women, the practice is not widely adhered to within the Conservative laity. However more and more congregations are building their own mikvaot, and observance is becoming more popular.

Reform and Reconstructionist Judaism

Reform and Reconstructionist Judaism do not hold the halachic requirements of mikveh the way orthodox Judaism does. However, there are growing trends toward using mikveh for conversions, wedding preparation, and even before holidays. While most Reform Jews will probably never see the inside of a mikveh, there are many (particularly converts) who will fulfill the mitzvah at least once in their lives.

Requirements during use of a mikveh

▼Mikveh in Herodium, late Second Temple period, approximately 69AD

The classical requirement for full immersion was traditionally interpreted as requiring water to literally touch every part of the body, and for this reason all clothing, jewellery, and even bandages must be removed; in a contemporary mikveh used by women, there is usually an experienced attendant, commonly called the mikveh lady, to watch the immersion and ensure that the woman has been entirely covered in water.

According to rabbinical tradition, the hair counts as part of the body, and therefore water is required to touch all parts of it, thus meaning that braids cannot be worn during immersion; this has resulted in debate between the different ethnic groups within Judaism, about whether hair combing is necessary before immersion. The Ashkenazi community generally supports the view that hair must be combed straight so that there are no knots, but some Black Jews take issue with this stance, particularly when it comes to dreadlocks.  A number of rabbinical rulings argue in support of dreadlocks, on the basis that

• dreadlocks can sometimes be loose enough to become thoroughly saturated with water, particularly if the person had first showered
• combing dreadlocked hair can be painful
• although a particularly cautious individual would consider a single knotted hair as an obstruction, in most cases hair is loose enough for water to pass through it, unless each hair is individually knotted

Allegorical uses of the term Mikveh

The word mikveh makes use of the same root letters in Hebrew as the word for "hope" and this has served as the basis for homiletical comparison of the two concepts in both biblical and rabbinic literature. For instance, in the Book of Jeremiah, the word mikveh is used in the sense of "hope," but at the same time also associated with "living water":

O Hashem, the Hope [mikveh] of Israel, all who forsake you will be ashamed ... because they have forsaken Hashem, the fountain of living water

Are there any of the worthless idols of the nations, that can cause rain? or can the heavens give showers? Is it not you, Hashem our God, and do we not hope [nekaveh] in you? For you have made all these things.

In the Mishnah, following on from a discussion about Yom Kippur, immersion in a Mikveh is compared by Rabbi Akiva with the relationship between God and Israel. Akiva refers to the description of God in the Book of Jeremiah as the Mikveh of Israel, and suggests that just as a mikveh purifies the contaminated, so does the Holy One, blessed is he, purify Israel.

A different allegory is used by many Jews adhering to a belief in resurrection as one of the Thirteen Principles of Faith. Since "living water" in a lifeless frozen state (as ice) is still likely to again become living water (after melting), it became customary in traditional Jewish bereavement rituals to read the seventh chapter of the Mikvaot tractate in the Mishnah, following a funeral; the Mikvaot tractate covers the laws of the mikveh, and the seventh chapter starts with a discussion of substances which can be used as valid water sources for a mikveh - snow, hail, frost, ice, salt, and pourable mud.
Source: http://en.wikipedia.org/wiki/Mikveh