According to Torah law, it is permissible for a man to marry as many women as he likes, on the condition that he is capable of supporting all of them at a respectable standard. There is a verse in Yeshayahu (4:1) which speaks of "seven women grabbing hold of one man," indicating that, strictly speaking, it is permissible for a man to marry seven women. Hazal, however, enacted that one should not marry more than four women. There is an obligation of Ona (intimacy with one’s wife), which applies at least once a week, and therefore, the Sages legislated that one should not have more than four wives, so he can fulfill his Ona obligation with each wife at least once a week.

Over eight centuries ago, there lived a great Torah leader named Rabbenu Gershom Me’or Hagola, who enacted a ban against marrying more than one wife. He saw that polygamy led to husbands failing to adequately support their wives, and to fighting and strife among a man’s wives. He therefore issued a formal "Herem" (ban) strictly forbidding marrying more than one woman. This ban is commonly known as "Herem De’Rabbenu Gershom," and was accepted throughout the Jewish communities of Germany, Russia, Poland and virtually all areas of Ashkenazic Jewry. It was also accepted among many Sephardic communities.

The Shulhan Aruch (Eben Ha’ezer 1) ruled that Rabbenu Gershom’s ban "expired" in the year 5000 (1240), and thus it no longer applies. Nevertheless, the Shulhan Aruch writes, it is proper for Jewish communities to issue strict, communal bans against polygamy. Therefore, even among Sephardic populations that did not accept Rabbenu Gershom’s ban, and even according to the view that the ban expired in 5000, marrying more than one woman is deemed improper, and communities are urged to take measures to prevent polygamy. Indeed, it seems clear from the Talmud that the Tanna’im and Amora’im all had only one wife. And even though there are stories of Tanna’im and Amora’im whose wives caused them aggravation, there is no record of any of them marrying an additional wife.

This is, of course, the accepted practice throughout the Jewish world. Irrespective of the ban on bigamy according to American civil law, it is deemed forbidden even by force of Halacha for a man to marry more than one wife. In fact, some have the custom to include in the Ketuba (marriage contract) a clause in which the groom promises the bride not to marry any additional wives.

There is some discussion among the Halachic authorities regarding the extent to which Rabbenu Gershom applied his "Herem." Namely, does it override a Misva? If a married man dies without children, for example, such that his brother has a Misva to marry the widow, but the brother is already married, should he marry the widow despite Rabbenu Gershom’s ban? Some authorities maintain that Rabbenu Gershom did not apply his enactment to cases where marrying a second wife fulfils a Misva, but the Rama (Rabbi Moshe Isserles of Cracow, Poland, 1525-1572) rules that one may not marry a second woman even if this fulfills a Misva. Thus, even if a man has been married to a woman for over ten years and she has not conceived, he may not marry a second wife to fulfill the Misva of Peru U’rbu (procreation).

There is, however, one exception to Rabbenu Gershom’s ban. He made a special provision for extraordinary situations where the Rabbis see a particular need to allow a man to marry a second wife, such as if the wife is extremely ill, Heaven forbid, and cannot function as a wife but also refuses to or cannot accept a Get (writ of divorce). In such a case, the husband should secure a letter signed by one hundred Rabbis, three of whom live in different communities, and all of whom are knowledgeable and proficient in Halacha, allowing him to override the ban and marry a second wife. This provision is called the "Heter Me’a Rabbanim" ("Permission of One Hundred Rabbis"), and has been used, albeit with great discretion, throughout the centuries.

In any event, it is clear that regardless of the technical issues surrounding the parameters of the "Herem De’Rabbenu Gershom" and whether it applies in Sephardic communities today, according to all views a Jewish man should only have one wife, except under extraordinary circumstances as discussed above.