Jewish men from the age of bar mitzvah (13), every weekday morning. Women are exempt, as tefillin is a time-bound commandment.

Why aren’t tefillin worn on Shabbat?

Tefillin are called a “sign”, and Shabbat and Yom Tov are themselves a sign between God and Israel, so tefillin are not worn then (Menachot 36b).

What is inside tefillin?

Four Torah passages on parchment: in the head tefillin each in its own compartment, in the arm tefillin all on one parchment.

How often must tefillin be checked?

There is no fixed period as for a mezuzah if they are kept well, but many have them checked by a scribe from time to time, and always if the boxes are damaged or got wet.

Tefillin – what they are and how they are worn תְּפִלִּין

Tefillin (phylacteries) are two black leather boxes containing handwritten parchments with four passages of the Torah. Jewish men put them on the arm and the head during weekday morning prayers.

The source

“Bind them as a sign on your hand, and they shall be as frontlets between your eyes” (Deuteronomy 6:8; also 11:18 and Exodus 13:9, 16). The four passages that mention the mitzvah are written inside: Exodus 13:1–10 and 13:11–16, Deuteronomy 6:4–9 and 11:13–21. The Talmud details how the boxes and straps are made (Menachot 34b–36a).

How they are put on

The arm tefillin first: on the upper arm opposite the heart (the left arm for a right-handed person), the strap wound seven times around the forearm.
Then the head tefillin: above the hairline, centred between the eyes; the knot sits at the base of the skull.
The remaining strap is wound around the hand and middle finger.
They are worn on weekdays in the morning prayer, not on Shabbat or Yom Tov (Menachot 36b).
Boys begin at bar mitzvah; many start practising a month or so before.

Sephardi and Ashkenazi customs

Ashkenazim stand for both tefillin, wind the arm strap outward and say a second blessing (“al mitzvat tefillin”) on the head tefillin; Sephardim sit for the arm tefillin, wind the strap inward and say one blessing for both. Hasidim and many Sephardim also put on a second pair, “Rabbeinu Tam” tefillin, which follow a different order of the passages (Menachot 34b, Rashi and Rabbeinu Tam). Most Ashkenazim outside Israel wear tefillin on Chol HaMoed; Sephardim, Hasidim and Israelis do not.

Sources

וּקְשַׁרְתָּם לְאוֹת עַל־יָדֶךָ וְהָיוּ לְטֹטָפֹת בֵּין עֵינֶיךָ
Deuteronomy 6:8

Talmud and halachic sources

Talmud, Menachot 34b–37b and 44a
Shulchan Aruch, Orach Chayim 25–45

Frequently asked questions

Who wears tefillin?: Jewish men from the age of bar mitzvah (13), every weekday morning. Women are exempt, as tefillin is a time-bound commandment.
Why aren’t tefillin worn on Shabbat?: Tefillin are called a “sign”, and Shabbat and Yom Tov are themselves a sign between God and Israel, so tefillin are not worn then (Menachot 36b).
What is inside tefillin?: Four Torah passages on parchment: in the head tefillin each in its own compartment, in the arm tefillin all on one parchment.
How often must tefillin be checked?: There is no fixed period as for a mezuzah if they are kept well, but many have them checked by a scribe from time to time, and always if the boxes are damaged or got wet.