Answer:
   
   In biblical times, polygamy was permitted. The Bible, in tolerating
   polygamy, gives evidence that the practice had long been an accepted
   social institution when these laws were written down. In the
   patriarchal age polygamy is regarded as an unquestioned custom. While
   the Bible gives a reason for the action of Abraham in taking Hagar for
   an additional wife and, in the case of Jacob, for having Rachel as a
   wife besides Leah, it only proves that polygamy as well as
   concubinage, with which it was always associated, was among the mores
   of the ancient Hebrew people (Gen. 16:1-4; 29:23-28). The same
   attitude is revealed in the episode of Abimelech and Sarah (Gen. 20:1-
   l3).
   
   Polygamy was such a well established part of the social system that
   Mosaic law is not even critical of it. We find only certain
   regulations with respect to it; as, for example, if a man takes a
   second wife the economic position of the first wife and of the
   children she bore must be secure; and, in the case of inheritance, no
   child of a subsequent marriage is to be preferred over a child from
   the first wife. Other regulations were that the high priest could have
   only one wife and that a king in Israel should not have too many wives
   (Lev. 21:13; Deut. 17:17; Ex. 21:10). The last injunction, however,
   was of no effect. David had seven wives before he began to reign in
   Jerusalem, and an extraordinary number of wives and concubines has
   been attributed to Solomon (II Sam 3:2- 5, 14; 5:13). In connection
   with David, the prophet Nathan did not denounce the king for adding
   Uriah's wife to those he already had but for the means he employed to
   secure her (II Sam. 12:7-15).
   
   However, if polygamy was not forbidden it was not directly sanctioned.
   It was a heritage from the past and it was left undisturbed. As the
   civilization of the people reached a higher form and, especially under
   the teaching of the prophets, their moral and religious consciousness
   developed, the polygamous system gradually declined. This is
   noticeable in Israel after the return from the Exile. We know that it
   survived into the Second Commonwealth, as evidenced in Christian
   writings (for why else would Jesus refer to the practice).
   
   According to the Talmud the right to a plurality of wives is conceded,
   but the number of legitimate wives, as in the Koran, is limited to
   four. The taking of additional wives is held as sufficient ground for
   divorce for a woman who had previously been the sole wife. Where a
   polygamous union exists, provision must be made for adequate
   maintenance of each wife as well as a separate domicile. Throughout
   the Talmudic age not one rabbi is known to have had more than one
   wife. Monogamy was held to be the only ideal legal union; plurality of
   wives was a concession to time and condition. At a later period
   Maimonides in his Mishneh Torah maintains, contrary to his personal
   opinion, that polygamous unions from a strictly legal point of view
   are permissible.
   
   About the year 1000, Rabbi Gershom ben Judah issued an edict (Herem
   de-Rabbenu Gershom) which was considered authoritative among Ashkenazi
   Jews. This edict substantially prohibited plural marriage. One
   exception was allowed: A man could marry more than one wife if he
   obtained the special permission of 100 rabbis in 3 countries.
   Originally, Gershom's ban was limited in time to the year 1260,
   however it has continued to be accepted by Jews of Europe and the
   Western World to this day. Rabbi Gershom's edict was followed less in
   sephardic countries: cases of polygamy were found in Spain as late as
   the 14th cent. That such cases were not rare may be inferred from the
   fact that in the Spanish communities the Ketubah, the document marking
   the betrothal, exacted that the man was not to take a second wife. The
   Islamic influence on the Jews in Spain was more or less pronounced
   until the expulsion at the end of the 15th cent.
   
   Nowadays, technically, polygamy is permitted among non-Ashkenazi
   (non-Northern European tradition) Jews and Ashkenazi Jews who obtain
   special permission of 100 rabbis (as in the case of (G-d forbid) a
   wife who becomes incapacitated). However, this is largely an academic
   question, because:
     * Most Jews live in countries that ban polygamy by civil law.
     * Most Jews still follow Rabenu Gershom's edict that banned
       polygamy.
       
   Yeminite Jews are a distinct case, being neither Ashkenazi or
   Sephardi. The Yemenite Jews were isolated from all Jewish people from
   the time of their exile in the middle of the first Temple period until
   recently. Yeminite Jews do not follow Rabbi Gersonm's edict, and
   believe that in some cases, the Torah even requires polygamy. An
   example cited is the case of "yebum", in which a man's brother dies
   and he must marry his wife, even if he is married already. As a
   result, some Yemenite Jews still take plural wives.
   
   Note: The Sephardic community in Israel has its own ban on performing
   polygamous marriages in Israel. In Israel, some Yemenites who came
   with more than one wife, still have them (including the last wave of
   immigration).