Chazal instituted that after a minyan prays Shemona Esreh the Shaliach Tzibbur should repeat Shemona Esreh aloud. Although the reason Chazal instituted this practice was in order to fulfill the obligation of those who didn't know how to say Shemona Esreh on their own, this practice is still an obligation for a minyan of people who are all capable of saying Shemona Esreh themselves. During Chazarat HaShatz there is a mitzvah of Kedusha described on its own page. During Chazarat HaShatz, the Cohanim perform the mitzvah of Birkat Cohanim which is discussed on its own page.
Proper behavior during Chazarat HaShatz
- When the Shaliach Tzibbur is saying Chazarat HaShatz, the congregation should listen and answer Amen. If there's not a minyan of people answering Amen it is like the Brachot that the Shaliach Tzibbur is saying are Brachot Levatala.
- According to Sephardim, when one hears Hashem's name in a Bracha one should answer "Baruch Hu UBaruch Shemo".
- One may not answer amen to a bracha once the chazan begins the next bracha. Thus, the chazan must wait for the people to answer before beginning the next bracha. 
- One who has the practice to answer Amen during chazarat Hashatz in places besides the end of a beracha (such as in Yaaleh Viyavo when we say zochrenu Hashem elokenu bo litova or in Barech Alenu when we say Shomra Vihatzlia shana zoh mikol davar rah) has on who to rely. 
Sitting or Standing
- Some say that the congregation should stand during the Chazarat HaShatz. Some have the practice to sit.
Doing other Things
- Those who wear Tefillin of Rashi and Rabbenu Tam should not take off their Tefillin of Rashi and put on the Tefillin of Rabbenu Tam during Chazarat HaShatz. 
- It is a grievous sin to talk during Chazarat HaShatz, even if one is not davening with that minyan. 
- It is improper to learn Torah or say other parts of davening during Chazarat HaShatz.
Practices of the Shaliach Tzibur
- According to the Sepharadic custom, at the beginning of the Chazarat HaShatz, the Shaliach Tzibur should say the pasuk "Hashem Sefatay Tiftach ..." aloud. According to the Ashkenazic custom, at the beginning of the Chazarat HaShatz, the Shaliach Tzibur should say the pasuk "Hashem Sefatay Tiftach ..." softly. 
- The Shaliach Tzibur should not start the next bracha until the majority of the congregation finished answering amen. 
If the Shaliach Tzibur made a mistake
- If the Shaliach Tzibur made a mistake during his silent Amida which would entail repeating his Amida, such as, forgetting to say "Yaale VeYavo ..." on Rosh Chodesh, he should not go back and repeat his silent Amida. Rather, he should continue with the Chazarat HaShatz, and when he comes to the point where he made a mistake, he should have in mind to fulfill his obligation. 
- If the Shalich Tzibur mistakenly forgot to say Kedusha in the Chazarat HaShatz, as long as he remembered before finishing the Beracha of Ata Kadosh he may go back, recite Kedusha, and continue again with "Ata Kadosh." 
- If the Shaliach Tzibur mistakenly skipped the Beracha of Ata Kadosh and finished the Beracha of Ata Chonen, he may still go back, recite Ata Kadosh, and continue again with Ata Chonen. 
- If the Shaliach Tzibbur makes a mistake in Chazara and forgets to say an addition has the same halacha as in individual who forgets that addition. The one exception to this is that if the Shaliach Tzibbur forgets to say Yaaleh VeYavo on Rosh Chodesh in Shacharit and he has completed the Chazarat HaShatz he does not have to repeat it. However, if he remembers before the end of the Chazara he should go back to Retzeh and add in Yaaleh VeYavo. 
- The same is true if on Shabbat or Yom Tov, meaning that if the Shaliach Tzibbur doesn’t mention any reference to Shabbat or Yom Tov in Chazara of Shacharit, but rather says a Chazara of a weekday Shacharit. If he completed the Chazara he doesn’t have to repeat. However, if the Shaliach Tzibbur remembered before the end of Chazara he should return to the beginning of Yismach Moshe (on Shabbat) or Atta Bechartanu (on Yom Tov). 
- A Shaliach Tzibbur who makes a mistake and skipped a Bracha of Shemonah Esrei (in Chazarat HaShatz) and when he is corrected he returns to where he made the mistake, he should continue to be Shaliach Tzibbur. However, if he skipped Birkat HaMinim (VeLamalshinim) the Shaliach Tzibbur should be removed immediately because there’s a fear that he is a heretic. 
- A Shaliach Tzibbur who makes a mistake and skipped a Bracha of Shemonah Esrei (in Chazarat HaShatz) and when he is corrected he is unable to return to where he made the mistake, another Shaliach Tzibbur should take his place. The next one should begin from the beginning of the Bracha where the mistake happened and if the mistake happened in the first three Brachot of Shemonah Esrei the Shaliach Tzibbur should begin from the beginning of Shemonah Esrei and if it was in the last three Brachot of Shemonah Esrei he should begin from Retzeh. 
When to start Chazarat HaShatz
- Some say that if the Shaliach Tzibbur is ready to start the Chazarat HaShatz and there's someone praying behind him, it's permitted to take the three steps back before beginning Chazarat HaShatz. However, some say that one should say Aseh HaShalom without taking the three steps back until after Chazarat HaShatz. 
- If there's minyan of ten people and one person is praying a lengthy silent Shemona Esheh, it's preferable to wait for him, however, if there are needs such as if the minyan will separate or if it's a bother to congregation to wait, they may begin Chazarat HaShatz even though one is still saying Shemona Esreh.
- The chazan should not speak between completion of his silent amidah and the start of the repetition of the amidah. Even learning Torah during that time is inappropriate as he should use the time to prepare himself to lead the prayer. 
- Shulchan Aruch 124:1
- Shulchan Aruch 124:3, Yalkut Yosef 124:1
- Shulchan Aruch 124:4, Kitzur Shulchan Aruch 20:2
- Shulchan Aruch 124:5
- Sh"t Rivivot Ephraim 1:88:1
- Yechave Daat 3:9
- Rama 124:4 writes that some say one should stand during Chazarat HaShatz. Kitzur Shulchan Aruch 20:1 writes that for someone for whom it is easy to stand should stand just as if one was actually saying Shemona Esreh oneself. See Mishna Brurah 124:20 who writes that the earlier practice was to listen to Chazarat HaShatz standing so that it was like one is actually praying through Shomea KeOneh, however, because of our sins in our days people do whatever they like and some sit. The Biur HaGra 124:4 (according to the Birkat Eliyahu) explains that the reason to stand is that Chazarat HaShatz was originally instituted in order to fulfill the obligation of one who doesn't know how to say Shemonei Esrei. Just like someone who didn't know how to say Shemonei Esrei would have to stand in order to fulfill his obligation by listening to the Chazara, so too the congregatoin should stand. The Kitzur Shulchan Aruch of Rav Rephael Baruch Toledano, Volume 1, Page 110 states that during the Chazarat Hashatz one should stand; nevertheless, one who is old or sick is permitted to sit.
- Kitzur Shulchan Aruch 20:1, Pri Megadim M"Z 34:2
- Shulchan Aruch 124:7, Mishna Brurah 124:27, Kaf HaChaim 124:37
- Sh"t Rivivot Ephraim 1:83
- Magen Avraham 124:8, Mishna Brurah 124:17, Kaf HaChaim 124:16, Kitzur Shulchan Aruch of Rav Rephael Baruch Toledano, Volume 1, Page 110. Shiur of Rabbi Shay Schachter on yutorah.org about learning during Chazarat HaShatz.
- Yalkut Yosef 124:1.
- Kitzur Shulchan Aruch 20:3, Sh"t Rivivot Ephraim 1:88:1
- Yalkut Yosef 124:8.
- Yalkut Yosef 124:10
- Yalkut Yosef 124:11
- The Gemara Brachot 30b quotes a Briatta which says that if one forgets to say Yaaleh VeYavo at Shacharit one doesn’t have to repeat Shemonah Esrei because one could make it up at Mussaf.
- Rashi (30b s.v. BeTzibbur) explains that the Gemara is discussing an individual who forgot to say Yaaleh VeYavo in Shacharit and the reason he doesn’t have to repeat it is because he will hear Yaaleh VeYavo from the Shaliach Tzibbur in the Chazara. The Tur 126 asks on this explanation because seemingly it doesn’t fit back into the words of the Gemara.
- Bahag (quoted by Rashi above) explains that the Gemara is discussing a Shaliach Tzibbur who forgot to say Yaaleh VeYavo in the Chazara of Shacharit and the reason he doesn’t have to repeat is because there would be an inconvenience to the congregation to make them hear another Chazara rather it’s enough that they will mention Rosh Chodesh in Mussaf. This explanation is brought in the Rif (Brachot 21a), Rabbenu Yonah (Brachot 21a s.v. UbeTzibbur), Rosh (Brachot 4:23), and Rambam (Tefillah 10:12). This is the basis for the ruling of Tur and S”A 126:3.
- The Bet Yosef 126:3 quotes a response of Rashi which says that the only case where one doesn’t repeat is if one finished the Chazara however, if one didn’t finish Chazara it’s not considered a bother to congregation to return to Yaaleh VeYavo. This is the ruling of S”A 126:3.
- The Rosh (Brachot 4:23) explains that this leniency only applies to Shacharit because the congregation will say Mussaf immediately afterwards as opposed to forgetting in Mussaf where it’s not sufficient not to repeat and depend on mentioning Rosh Chodesh in Mincha because Mincha is not said immediately afterwards.
- However, the Smag (quoted by Tur 126:3) says that it also applies to Mussaf; if the Shaliach Tzibbur didn’t mention Rosh Chodesh in the Chazara of Mussaf then he is not obligated to return because he can make it up in Mincha. However, the Tur argues strongly that this is not found in the Gemara. The Bet Yosef 126:3 quotes the Smag saying that this actually was one version of the text of the Gemara, nonetheless, the Bet Yosef concludes that we hold like the Tur because that is the text of the Rosh 4:23, Rif 21a, and Rambam 10:12. The S”A 126:3 rules like the Tur that there’s no leniency by Mussaf.
- [Actually, our version of the Gemara says that the same is true of forgetting Yaaleh VeYavo in Mariv and Mussaf because one could make it up in Shacharit and Mincha, respectively. However, Tosfot 30b writes that the version which extends this idea to Mariv is incorrect because the later Gemara gives a different reason why one who forgets to say Yaaleh VeYavo at Mariv doesn’t have to repeat Shemonah Esrei. Additionally, Hagahot HaGra ( Ibid.) writes that the Rif and the other poskim don’t have the text which extends this idea beyond Shacharit.]
- For background see above footnote. The Smak (quoted by the Tur 126:3) writes that the leniency of the Gemara, that if the Shaliach Tzibbur forgets to mention Rosh Chodesh in Chazara of Shacharit on Rosh Chodesh he doesn’t have to repeat the Shemonah Esrei applies equally to Shabbat and Yom Tov. [The Mishna Brurah 126:14 explains the case is that the Shaliach Tzibbur said a Chazara of a weekday Shacharit in place of Shabbat or Yom Tov.] The Tur agreed with the Smak. The Rama 126:3 rules like the Smak. The Mishna Brurah 126:15 writes that even though some argue the halacha follows the Rama.
- However, the Smag (quoted by the Tur 126:3) and Bet Yosef 126:3 based on the language of the Gemara and Rambam disagree and say that the leniency of the Gemara is limited to Rosh Chodesh and doesn’t apply to Shabbat and Yom Tov. Nonetheless, Kaf HaChaim 126:19 writes that if the Shaliach Tzibbur finished the Chazara one may rely on the Rama because of delaying the congregation.
- Tur and S”A 126:1
- Tur and S”A 126:1
- Avnei Yishpeh 5:15
- Halichot Shlomo (Tefillah pg 289)
- Tzitz Eliezer 12:9
- Sh"t Rivivot Ephraim 1:50:16