According to Jewish law, the mikve is used by women who have the status of nidda (required separation during and after menstruation and following childbirth) and by converts to Judaism as the final stage of the conversion process.
The source of the rules of nidda are two verses in Leviticus: "A woman who has a flow of blood in her body shall be a nidda for seven days, and all who touch her shall be ritually impure until sundown"; and "A woman in the ritually impure state of nidda, you shall not approach for sexual relations" (Leviticus 15:19 and 18:19). Halacha has added an extra five days to this time.
In addition, the mikve is used voluntarily by some men before certain festivals and in the lead-up to Shabbat. It is also used to immerse new dishes before they are used in the home.
The mikve is a ritual immersion pool containing an initial volume of rainwater and frequent additions of clean tap water. To be considered kosher, the mikve must consist only of non-flowing water and be touching an adjacent pool of accumulated rainwater. That is why each mikve contains a bor (pit) of natural rainwater built directly into the ground. The mikve should consist of at least 40 se'a of water (331 liters) so that a woman can fully immerse her entire body.