- Chazal viewed the recitation of Amen very highly. In fact, Chazal tell us that responding Amen is of greater significance than reciting the bracha. The failure to recite Amen is considered a gross transgression, while responding Amen with great concentration opens the gates of Gan Eden. 
- The letters of Amen are the root letters of the word Emunah, belief or trust. By responding Amen one declares: "I believe in the bracha that I have just heard and I affirm its truth." Additionally, when responding Amen one should have in mind the beginning of the bracha, "Baruch Atta Hashem", that Hashem's name is Blessed. 
When to Answer Amen
- If a person hears a bracha made properly by a Jew there is an obligation to answer Amen. This obligation is derived from the verse כי שם יהוה אקרא הבו גדל לאלהינו. 
- One should answer Amen to any blessing one hears whether one wishes to fulfill an obligation or not. It is proper to answer Amen after a Tefillah or bracha even if it does not contain the name of Hashem (Shem UMalchut) for example: the Mi SheBerach (מי שברך), Harachaman (הרחמן) in benching, and some add “Makom Yenachem Etchem…”. 
- One may answer Amen to a Yehi Ratzon a Jew says even if the Yehi Ratzon did not have Hashem's name in it.
- Amen must be said within Toch Kedi Dibbur (2-3 seconds) of the bracha or if one is in a congregation one can say Amen until the majority of the congregation has finished saying Amen. 
- When hearing a bracha over a microphone, if one is in the same room one may answer Amen. If not, some Poskim hold one may not answer Amen, while others hold one may answer but one cannot fulfill any obligations in this manner. Similarly, some say that one should answer Amen to a bracha one heard over the phone
- One who is in the middle of learning does not need to answer Amen to a bracha or to Kaddish.
Answering Amen to One's Own Bracha
- While Ashkenazim only answer Amen to their own bracha at the end of Boneh Yerushalayim in Birkat HaMazon, the Sephardic tradition is to answer Amen in any major instance of making a series of brachot, such as at the end of Yishtabach, Sim Shalom, Hallel, the Haftarah brachot, and Hashkiveinu. This does not include Brachot recited before performance of a mitzvah or eating, as that would constitute a Hefsek.
- If two people make the same bracha at the same time, neither should respond Amen, because it seems as if one is responding Amen to one's own bracha. Also see the Answering_Amen_to_Your_Own_Bracha page.
How to Properly Answer Amen
- The proper intention of the word Amen changes with the bracha. When answering Amen to Birchot HaMitzvah or Birchot HaNehenin, one's intention should be to affirm the truth of the bracha and one's belief in it. When answering to Birchot HaShevach, one should have in mind that one is affirming the truth of that praise. When responding Amen to tefillot one's Amen should be a request of Hashem to fulfill that prayer. 
- One should not answer Amen louder than the bracha itself unless one is doing so in order to motivate others to answer Amen.
- If one hears two brachot, one should answer Amen VeAmen rather than Amen Amen.
Common Mistakes When Answering Amen
- One must make sure to pronounce the kamatz under the Aleph of Amen, otherwise it is considered an Amen Chatufa. Additionally, Amen Chatufa includes when someone responds Amen before the bracha has been completed. 
- One should ensure to pronounce the nun of Amen, otherwise it is considered an Amen Katufa. 
- One should be sure to answer Amen slowly. One's Amen should take as long as it would normally take to say El Melech Neeman, but not overly protracted.
- If one is obligated in a certain bracha and someone is reciting the bracha on one's behalf, if one does not hear its recitation one should not answer Amen, as this would be considered an amen yetomah.  Alternatively, an Amen yetomah is when one waits before responding. Therefore, when one hears a bracha, one should respond immediately. 
- If one did not hear a bracha tat was made, one should not answer Amen, otherwise it would be considered an Amen Yetoma. In certain situations, hearing part of the bracha may be enough.
- Preferably one should hear the bracha of the Shaliach Tzibbur and answer amen, however, after the fact, even if one did not hear the bracha but knows which bracha was made, one may answer Amen. 
Answering to Kaddish
- One who answers Amen yehei shemei rabba with all one's strength annuls any bad decrees. 
Answering Baruch Hu UBaruch Shemo
- When hearing the words Baruch Atta Hashem, one should answer "Baruch Hu UBaruch Shemo". The Chazzan should pause to give the congregation a chance to respond "Baruch Hu UBaruch Shemo".
- One should not say "Baruch Hu UBaruch Shemo" when one intends to fulfill an obligation by listening to someone else's bracha or when speaking is prohibited such as by Pesukei DeZimrah and Birchot Kriyat Shema. 
- Gemara Brachot 53b, Chaye Adam (Klal 6:1), Gemara Shabbat 119b, Rashi Shabbat 119b s.v. BeChol
- Shulchan Aruch 124:6, Mishna Brurah 124:24
- Shulchan Aruch 215:2 based on the Gemara Yoma 37a, Rambam Hilchot Brachot 1:13, Mishna Brurah 215:8, Kaf Hachayim 124:30.
- Shulchan Aruch 124:6, 189:5, 215:9; Mishna Brurah 215:9; Vezot HaBracha pg 188 in name of Rav Shlomo Zalman
- Nitei Gavriel Rosh Hashana 29:23
- Rama 124:8 says that the Amen must be said immediately after the bracha and Mishna Brurah 124:34 explains that this means Toch Kedi Dibbur. Vezot HaBracha (pg 189) agrees.
- Vezot HaBracha (pg 189) in name of Rav Shlomo Zalman
- See Brachot Through a Microphone
- Rav Moshe Feinstein (Igrot Moshe 4:91:4)
- Halichot Shlomo Tefilla 9:6 writes that one need not answer to a bracha or a Kaddish, but one should respond to Kedusha and Modim together with the tzibbur if one is in the same room so that one does not appear to be breaking off from the tzibbur.
- See Answering Amen to Your Own Bracha
- Mishna Brurah 51:3
- Shulchan Aruch 124:6, Mishna Brurah 124:10,25, Vezot HaBracha pg 188
- Gemara Brachot 45a, Shulchan Aruch 124:12, Mishna Brurah 124:47, Kitzur Shulchan Aruch of Rav Mordechai Eliyah 6:10
- Mishna Brurah 124:25
- Shulchan Aruch 124:8, Beiur Halacha 124:8 s.v. Kriyat, Kitzur Shulchan Aruch 6:10. Brachot 47a warns against answering with an Amen chatufa, “a snatched amen.” Rashi s.v. “chatufa” explains that this refers to one who hurries the vowelization of the first letter, not enunciating the kamatz under the alef and instead pronouncing a chataf. The Aruch (Erech "Amen") explains that chatufa refers to one who answers amen before the bracha is finished. Both of these definitions are quoted in Shulchan Aruch. 124:8 as well as Kitzur Shulchan Aruch 6:10.
- Shulchan Aruch 124:8, Beiur Halacha 124:8 s.v. Kriyat. Brachot 47a also warns against answering an “amen ketufa,” “a cut-off amen.” Rashi explains that one "curtails" the amen by omitting the nun. Alternatively, the Aruch (Erech “Amen”) explains that “amen ketufa” refers to one who pauses in between the two syllables, dividing amen into two words. Again, the Shulchan Aruuch and Rama 124:8 cite both opinions.
- Shulchan Aruch 124:8, Mishna Brurah 124:36, Brachot 47a
- Berachot 47a warns against answering an amen yetoma. Rashi (s.v. Yetoma) and Tosfot (s.v. Amen Yetoma) explain that the respondent has not heard the actual bracha, but joins the congregation and responds amen. This does not contradict the practice of the large synagogue in Alexandria (Sukkah 51b) where flags were waved at the appropriate spots to indicate that Amen should be recited. In that case, those answering knew they were responding to a bracha and to which one they were responding, even though they could not hear the actual voice of the chazan.
Alternatively, Tosfot Sukkah 52a bring the approach of of Rabbeinu Nissim Gaon that this is only a problem if one wants to fulfill one's obligation by hearing the bracha. In Alexandria they could respond because they had already fulfilled their obligation. This is the approach of Rabbenu Yonah 34b s.v. “Ein Onin” and Rambam Brachot 1:14 as well. See Rav Daniel Feldman (“An Orphaned Amen” on Yutorah) who quotes Rav Soloveitchik’s (Reshimat Shiurim to Sh’vuot 36b, p 116) explanation of this machloket.
Shulchan Aruch 124:8 holds like Rabbenu Yonah, while the Rama writes that some are strict (in accordance with Rashi and Tosfot in Brachot) that even if one is not obligated in a bracha, one should not respond unless one knows which bracha was recited. Biur Halacha s.v. “yesh machmirin” says that although many acharonim disagree with the Rama, it is better to remain passive and avoid the possibility of reciting an Amen yetomah, due to its severity.
While the Ben Ish Chai Teruma 13 only mentions the opinion of the Shulchan Aruch, Yalkut Yosef (Tefilla v. 2, p. 163) writes that one should be concerned for that opinion of the Rama and not respond to a bracha if he does not know which one is being said. Kaf Hachaim 124:47 agrees.
Interestingly, Rav Moshe Feinstein (Iggerot Moshe YD 4:61:16) writes one needs to know which part of Kaddish one is answering.
- A third explanation of Amen yetomah is brought by the Avudarham (quoted in Beit Yosef 124). He says that saying an Amen yetomah means responding Amen long after the bracha was recited. The Rama 124:8 cites this interpretation. Mishna Brurah 124:34 explains that this means that one should respond within Toch Kedi Dibbur. Vezot HaBracha (pg 189) agrees.
- The Mishnah Brachot 51b states that although regarding a non-Jew one can only answer Amen if one hears the entire bracha, one can answer Amen to a partial bracha from a Jew. This is codified as halachah in Shulchan Aruch 215:2. Tur 215, Rosh Brachot 8:5 and Talmidei Rabbenu Yonah explain that one can respond Amen to a Jew’s bracha as long as one heard everything from Hashem's name onwards (i.e. the entire blessing except for the words "baruch atah". Rashi and Or Zarua, however, hold that one can respond Amen as long as one heard the very end of the blessing (i.e. "borei minei Mezonot"). Mishna Brurah 215:6 records both opinions.
- Concerning Chazarat HaShas the Mishna Brurah 124:33 writes that it is preferable to hear the bracha from the Shaliach Tzibbur since some say that since it is a rabbinic institution it is considered a bracha one is obligated in, however, after the fact, one can answer as long as one knows which bracha was made (because one knows which bracha the tzibbur was up to even though one did not hear the bracha). Vezot HaBracha pg 189 agrees. Yalkut Yosef (Tefillah vol 2 pg 163, siman 124 note 8) rules that one should be strict like the Rama, except that by Chazarat HaShas if one already prayed one may answer as long as one knows which bracha is being made.
- Gemara Shabbat 119b
- Shulchan Aruch 124:5, Mishna Brurah 124:21, 22, Maharam Shik 51, Igrot Moshe 2:94