The Gemara in Masechet Kiddushin (36) addresses the question of whether women are included in the obligation of Tefillin, and establishes that they are exempt. As the Misva of Tefillin does not apply on Shabbat, on holidays, or – according to some opinions – at nighttime, it falls under the category of "Misvot Aseh She’ha’zman Gerama" – a Misva that is restricted to specific time-frames. There is a famous rule that Misvot included under this category are not imposed upon women (with several exceptions), and thus woman are exempt from the Misva of Tefillin, just as they are exempt from the Misvot of Shofar, Lulab and Sukka.
The question then becomes whether a woman who desires to perform this Misva is allowed to do so. Generally speaking, a woman who chooses to perform a Misva from which women are exempt is not only allowed to perform the Misva, but also granted reward for the Misva. And thus, most women today make a point of coming to the synagogue on Rosh Hashanah to hear the Shofar blowing, and of eating in the Sukka on Sukkot, even though they are exempt from these obligations. According to Sephardic custom, based upon the ruling of the Shulhan Aruch, women do not recite a Beracha when performing a Misva from which they are exempt, but they are certainly permitted to perform such a Misva, and even receive reward for it.
At first glance, this should apply to Tefillin, as well, and we should permit women to wear Tefillin if they so desire, albeit without a Beracha.
For several reasons, however, Tefillin marks an exception to this rule, and the consensus of Halachic authorities forbids, or at least discourages, women from wearing Tefillin. Firstly, the Torah in the Book of Debarim forbids women from wearing clothing that is unique to men – "Lo Yihye Keli Geber Al Isha" – and the Targum Yonatan Ben Uziel interprets this prohibition as referring to Sisit and Tefillin, which are worn by men. Hence, Tefillin differs from Misvot such as Shofar, Lulab and Sukka in that it involves wearing a type of "garment," and it is therefore subject to the prohibition of "Keli Geber." The Ben Ish Hai (Rav Yosef Haim Baghdad, 1833-1909), in Parashat Lech-Lecha, cites this source as a reason for women not to wear Tefillin, which is considered men’s "attire."
Another reason is mentioned by Tosafot (Medieval French and German Talmudists), in Masechet Erubin, and by the Kolbo (an anonymous Halachic work), as cited by the Bet Yosef (Orah Haim 38). The Misva of Tefillin, unlike most other Misvot, requires "Guf Naki" – an especially strict standard of cleanliness, and thus men must ensure that their bodies are perfectly clean when they wear Tefillin. Women, who occasionally become Nidda, cannot necessarily maintain the required standard, and for this reason, according to Tosafot and the Kolbo, they should not wear Tefillin. Indeed, the Rama (Rabbi Moshe Isserles of Cracow, 1525-1572) rules that if a woman wishes to wear Tefillin she should be instructed not to do ("Yesh Li’mhot Be’yadam"), and the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) explains this ruling based on the requirement of "Guf Naki."
The Ben Ish Hai in Parashat Lech Lecha cites the reason of Targum Yonatan and adds a third consideration, namely, the issue of "Mehzeh Ke’yuhara" ("It appears arrogant"). It is forbidden to take upon oneself stringent measures that are not required according to strict Halacha and are not conventionally observed. One who takes on such a practice makes himself stand out, and this is considered an act of hubris, which is forbidden. Thus, for example, it would be inappropriate for a man to go about throughout the day wearing Tefillin, unless he is widely recognized and respected as an especially devout Sadik. Torah giants such as Hacham Ovadia Yosef did not wear Tefillin all day, and thus if an ordinary person would take on this practice, he would appear arrogant and presumptuous. The Ben Ish Hai applies this concept to women wearing Tefillin, and writes that since women do not customarily wear Tefillin, and doing so would appear strange, a woman who wears Tefillin comes across as arrogant. It is therefore forbidden even if not for the aforementioned concerns of "Keli Geber" and "Guf Naki."
Some might argue that women should be allowed to wear Tefillin nowadays in light of the significant changes we have witnessed in women’s participation in religious life. Women study Torah to a far greater extent than in the past, and many women have achieved proficiency in Halacha and are even able to offer Halachic guidance. It must be noted, however, that even those Halachic authorities who have lent support to these changes do not propose that women wear Tefillin, nor do the schools that teach women advanced Halacha encourage them to wear Tefillin. Rav Yehuda Henkin, a contemporary Torah scholar who has strongly supported the new norms regarding women’s Torah education, writes explicitly in his work Beneh Banim (vol. 2) that women should not wear Tefillin. Responding to a woman who asked whether she may wear Tefillin, Rav Henkin urged her to "find different ways to connect herself to G-d."
We should also note that the Shilteh Ha’gibborim, who lived around the time of Maran, writes (Rosh Hashanah 9) that it is forbidden for a woman to wear Tefillin because this is "Mi’derech Ha’hisonim" ("the ways of the external forces"). It is not clear what kind of "Hisonim" he refers to, but in any case, this is his unequivocal Halachic ruling.
Indeed, the general consensus among the Halachic authorities is that women should not wear Tefillin, as mentioned by the Kaf Ha’haim (38), and by Hacham David Yosef, in his Halacha Berura. This position is also presented by the Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908), who notes that it is not even self-evident that we should permit men to wear Tefillin, due to the difficulty entailed in maintaining proper standards of cleanliness; we certainly should not go even further and permit women, who are exempt from this obligation, to wear Tefillin. The Aruch Ha’shulhan further notes the Gemara’s comment in Masechet Erubin (96) that Michal, daughter of King Shaul, wore Tefillin. This is the only mention anywhere throughout the Talmud of a woman wearing Tefillin, and clearly represents the exception, rather than the rule. Several reasons have been suggested for why she was allowed to wear Tefillin, such as she was exceptionally righteous, and she may have been uniquely capable of maintaining proper standards of cleanliness. Some have also pointed to the fact that she did not have children, which may have positively affected her ability to maintain required standards of "Guf Naki." In any event, the Talmud Yerushalmi and Pesikta comment that Michal’s contemporaries protested her wearing of Tefillin. Clearly, then, this cannot serve as a precedent for allowing women to wear Tefillin.
In conclusion, it is important to observe that among Orthodox circles, very few women have expressed serious interest in wearing Tefillin. This is prevalent among movements that are not committed to traditional Halachic observance, and among those who seek to promote feminism and abolish Halachic distinctions between men and women. Obviously, only G-d can judge people’s intentions, and we do not claim to know the motives of every woman who desires to wear Tefillin, but there are, unfortunately, those who vocally seek to challenge Halachic norms, and some express the desire to wear Tefillin to pursue this agenda. Even ignoring all the factors discussed above, it is clear that wearing Tefillin to promote an ideology is forbidden. If a woman is driven to wear Tefillin by a desire to correct what she perceives as Judaism’s injustices against women, then this is clearly not legitimate. The Mishna in Abot warns, "De’ishtamash Be’taga Halaf" – "One who makes use of the crown [of Torah] will be done away with." We are not entitled to use Torah and Misvot to advance personal agendas. The Beneh Yisaschar (Rav Tzvi Elimelech Shapiro of Dinov, 1783-1841) notes on this statement from the Mishna that the middle letters of the word "Be’taga" are "Tav" and "Gimmal," which have a combined numerical value of 403 – which is also the total number of words written on the parchment in the Tefillin. Moreover, the first and last letters of this word ("Bet" and "Alef") spell the word "Bo," which is the name of the Parasha in which the first two sections of the Tefillin appear in the Torah. And thus the Mishna is warning against using the Misva of Tefillin – "Be’taga" – for one’s own personal interests.
This, of course, is from an ideological standpoint. But as we have shown, even from a strictly Halachic perspective, it is improper for women to wear Tefillin, as ruled by the Rama, Kaf Ha’haim, Aruch Ha’shulhan and many others.