Leaning during the Seder
During the Seder we need to feel that we are free and do so by leaning in a comfortable and royal position. Some explain that this is a Biblical mitzvah, while others explain that it is only a rabbinic detail of the mitzvot of the night.
Leaning during the Seder
- There is an obligation to lean when one drinks the four cups of wine and eats the Matzah of Motsei Matzah, Korech, and Afikoman. 
- Leaning shouldn't be done on one's back, front, or to the right.  A lefty should also lean to the left. 
Leaning during Karpas
Who is obligated to lean?
- In the time of the Gemara women were exempt from leaning unless they were an Isha Chashuva (a woman of stature).  Even though nowadays all women are considered women of stature, Ashkenazi women don't have to lean because they can rely on the Ravyah.  However, the practice of Sephardic women is to lean. Yet Bedieved they don't need to repeat eating or drinking. 
- Even though Ashkenazi women are exempt from leaning, if they drink the wine or eat the Matzah standing they are not Yotzei. 
- A student in front of his Rabbi (even if it's not his Rav Muvhak, primary teacher), or a student in front of a very distinguished Rabbi even if he's not his teacher shouldn't lean unless they are given permission. 
- A son in front of his father should lean even if your father is also your teacher. 
- If a student went ahead and leaned even though he wasn't supposed to and the Rabbi was silent, the silence is understood as permission. 
- Lechatchila a student shouldn't ask his rabbi for permission to lean. 
- If a student is eating the Seder at another table but can be seen by his Rabbi, some authorities permit, but one should be strict not to lean unless one gets permission, especially if it's one's Rav Muvhak. 
- A son in front of his father should lean but a son-in-law in front of a father-in-law who is his teacher shouldn't lean. 
- A mourner for twelve months for a parent or within shloshim should lean.  Some hold that although the mourner should lean he shouldn't lean on a couch.
One who forgot to lean
- According to Sephardim, if one forgot to lean one has to lean again, even the third and fourth cup for which one shouldn’t drink afterwards. However, Ashkenazim hold that one shouldn’t drink the third or fourth cup again because one can’t drink after the third and fourth cup. Additionally one should have in mind before making the bracha to repeat the first cup if one forgets to lean. If one didn’t have such an intent, one shouldn’t repeat the first cup. 
- Bedieved, after the fact (when it can not to repeated) according to Ashkenazim one has fulfilled their requirement. 
- If one forgot to lean while eating Matzah (of Motzei Matzah) one must repeat eating a Kezayit of Matzah. 
- If one forgot to lean during Afikoman one should Lechatchila eat it again while leaning. However if it’s difficult to eat another Afikoman or one already said Birkat HaMazon one can rely on the lenient opinions and not eat Afikoman again. 
- The shamash (servant) is obligated in leaning. 
- Rambam Matzah 7:6-7 implies that leaning fulfills the Biblical mitzvah of Zachor Et Yom Tzetacha Memitzrayim. Midrash Rabba Bishalach learns the idea of leaning from the pasuk of Vayasev Elokim (Shemot 13:18).
- Chazon Ovadia citing Rashbam Pesachim 99b and others.
- Concerning wine requiring leaning, Pesachim 108a concludes that all four cups of wine need leaning. This is also the opinion of Rambam Hilchot Chametz UMatzeh 7:8 and he adds that whoever leans for the other eating and drinking of the night is praiseworthy; see S”A 473:2. The gemara Pesachim 108b says matza requires leaning while the marror doesn't. S”A 475:1 rules that Matzah of Motsei Matzah and Korech should be eaten while leaning. S”A 477:1 rules that Afikoman should be eaten while leaning. Tur in name of the Avi Ezri (Siman 525), Mordechai 34b in name of Ravan, and Hagahot Maimonin name of the Ravyah 7:2 hold that nowadays it's not the practice of royalty to lean and so there's no halacha that one has to lean during the Seder. Hagahot Maimon says that the Ravyah's opinion is a minority opinion and all other poskim argue on it. Nonetheless, Rama 472:2 rules that Bedieved one can rely on the Ravyah and fulfill the mitzvah without leaning. Shulchan Aruch who leave out this opinion clearly holds that there's a requirement of leaning nowadays. This is also the opinion of Sh"t Chazon Ovadyah 13. Rav Shlomo Zalman Auerbach in Halichot Shlomo Pesach 9:20 footnote 44 rules that one should only lean when he is actually eating or drinking but it is disrespectful to lean during the berachot.
- Pesachim 108a says that it's not considered leaning to lean on the "Perakdon" which some explain as on one's back (Tosfot 108a) and some explain as on one's front (Tosfot Niddah 14a in name of Aruch). Thus, the Rambam 7:8, Tur and Shulchan Aruch 472:3 rule that one can't lean to the back or front.
- The Gemara says that one can't lean on the Perakdon or to the right, and adds that leaning that way would be dangerous because the food can go down the windpipe and cause choking. Rashi explains that the danger is referring to leaning back and Rashbam explains that the danger is referring to leaning to the right. According to Rashi(quoted by Rashbam) that the only reason not to lean to the right is because one has to use one's right hand to eat, a lefty should have to lean to the right and eat with his left. According to the Rashbam who says that there's danger in leaning to the right, a lefty also should lean to the left. This Nafka Minah is brought down by the Rosh Pesachim 10:20 and Tur 472:3. Terumat HaDeshen 1:136 rules that the reason of a potential danger trumps the reason of that it's usual for a lefty to eat with his left hand. This is also the opinion of the Shulchan Aruch 472:3 that a lefty should also lean to the left.
- The Gemara 108a says that women don't have to lean. Rashbam explains that the reason is because it's not respectful of a wife to lean in front of her husband. Additionally, Rav Achai Goan in Shiltot (Tzav 77), (the Rif according the Aruch HaShulchan 472:6), and Tur 472:4 in name of the Bahag say that the reason is because it's just not proper for women to lean. According to both explanations it's understandable why the gemara says a women of stature is obligated to lean; according to the Rashbam it's because she's important and deserves respect and according to the Shiltot it's because it is her practice to lean. [A third reason is that of the Rokeach (quoted by the Chok Yacov 472:8) that a husband isn't concerned with his wife leaning if she's a woman of stature]. A clear Nafka Minah explained by the Tur is a widow or divorcee, according to the Rashbam should lean and according to the Shiltot shouldn't lean. Shulchan Aruch 472:4 implies that he holds like the Shiltot because he doesn't make any distinctions. So writes the Pri Chadash 472:4 and Birkei Yosef 472:3 that we pasken like the Shiltot. Implied by the Tur's Nafka Minah is that not married women aren't obligated according to everyone. Chiddushei Rav Shlomo Zalman Aurbach to Pesachim 108a explains that since most women are exempt because they are married, the Rabbis didn't establish an obligation on non-married women either.
- Rabbenu Yerucham (Section 5 pg 42d) and Mordechai (Arvei Pesachim 37d) in name of Tosfot say that all women nowadays are considered chashuv. [Rav Avraham Dov Auerbach (quoted in Halichot Shlomo Pesach pg 235) explains that this ruling of Tosfot is based on the edict of Rabbenu Gershom that it is forbidden for men to marry two women. (this would explain why Shulchan Aruch left out this ruling considering that Sefardim didn't accept the edict of Rabbenu Gershom).] This is also the opinion of the Tur and Rama 472:4 that all women nowadays are obligated to lean. Nonetheless, Rama concludes that the practice is that women don't lean because they rely on the Ravyah that nowadays there's no obligation to lean at all. [Aruch HaShulchan 472:6 asks if women rely on the opinion of the Ravyah why don't men also? Therefore Aruch HaShulchan argues that the lenient practice of women is based on the opinion of the Shiltot that a women is only considered a women of stature if it is her practice to lean and since it's not the practice of women to lean they are exempt. See Rav Shlomo Zalman Auerbach in Halichot Bat Yisrael 23:50 who defends the Rama saying that since the obligation of women was based on Minhag and not a real obligation they can rely on the Ravyah.] In conclusion, Ashkenazi women are exempt from leaning because they rely on the Ravyah. Sefer HaMapah and Levush write that such was the minhag of Ashkenaz. Rav Shlomo Zalman Auerbach in Halichot Shlomo Pesach 9:19 as well as Rav Moshe Shternbuch Moadim uzmanim 3:257 both write that a woman should try to get a comfortable chair as her expression of freedom.
- Seemingly according to the reason of the Shiltot (which is the reason we pasken, see previous notes) Sephardic women should not be considered Chasuv because it's not their practice to lean all year. However, Sh"t Chazon Ovadyah 14 writes that Sephardic women should lean because a husband isn't concerned with that behavior. One reason is that of the Birkei Yosef 472:3 who writes that since men and women are equal in the fact that they don't lean during the year, women are equal to men on the night of pesach that they are obligated to lean. A second reason to obligate women in leaning is according to the reason of the Rokeach that they are only exempt if their husbands are concerned with this practice and it offends them and nowadays all men aren't bothered by this behavior. A third reason is according to the Rabbenu Manoach that women are only exempt because they are involved in preparing the rest all the parts of the meal (similar to the patur of mitzvot aseh shezman grama) and since nowadays women prepare the entire meal beforehand or they have helpers they are obligated in leaning (so writes the Shulchan Gavoha 472:11). Pesach Meuvin (Siman 170; author of Knesset Gedolah http://www.hebrewbooks.org/pdfpager.aspx?req=10568&st=&pgnum=37) writes the minhag of Sephardic women was to lean. Sh"t Chazon Ovadyah writes that Bedieved one can combine the reasons of the Rama with the reasons that they might not be considered Isha Chashuva (a Sfak Sfeika) to be lenient. See: http://www.dailyhalacha.com/Display.asp?PageIndex=5&ClipID=854
- Rav Shlomo Zalman Auerbach (quoted in Halichot Shlomo Pesach pg 235) writes that even though women are exempt from leaning they are still obligated in the law of acting as free people and royalty, therefore if they eat while standing which is totally a lack of acting like royalty they have not fulfilled their obligation. One can also make such a implication from the words of the Ravyah 525 that "the practice of leaning was replaced by sitting".
- Shulchan Aruch 472:5
- Shulchan Aruch 472:5. Magen Avraham 472:6 explains that the father is assumed to forgo on his respect.
- Bet Yosef 472:5 writes that the Rabbi's silence is interpreted as permission to lean. Mamer Mordechai 472:2 comments that this is only true Bedieved but lechatchila a student shouldn't lean in the Rabbi's silence. This is also the opinion of the Darkei Moshe 462:3, Chok Yacov 472:11, Mishna Brurah 472:17, and Chazon Ovadyah (Pesach vol 2 pg 7).
- Chiddushei Rabbi Shlomo Zalman Auerbach on Pesachim 108b quoted in Halichot Shlomo pg 233 holds that it's a little disrespectful to ask to lean in front of your rabbi because the institution of lean was meant to show freedom and showing respect for one's rabbi overrides that and so it's proper not to ask permission.
- Rama 472:5 rules in name of the Mahariv that a student can lean in front of his Rabbi if the student isn't eating at the same table. Pri Chadash argues that anytime the student is in sight of his Rabbi he can't lean. Mishna Brurah (Shaar Tzion 472:22) argues that perhaps the Mahariv is lenient because of his opinion that a student in front of his Rabbi is exempt but is allowed, however since we hold like the Darkei Moshe that a student in front of his Rabbi is forbidden to lean perhaps in this case one can't lean. Mishna Brurah quotes the Pri Megadim that in such a case it's preferable to be strict and get permission. Chazon Ovadyah (Pesach vol. 2 pg 8) suggests that by a Rav Muvhak it's forbidden like the Pri Chadash and for one's Rabbi who isn't one's Rav Muvhak it's permissible like the Mahariv.
- Chiddushei Rabbi Shlomo Zalman Auerbach on Pesachim 108b quoted in Halichot Shlomo pg 233 writes that since a father is comfortable with his son leaning in front of him it's not considered disrespect, however a father-in-law doesn't have that same relationship with his son-in-law.
- Chazon Ovadia Hilchot Pesach page 9, Chok Yakov 472:5
- Bach 472:2 and Bach teshuva chadashot 5 writes that a mourner within 12 months may not lean since it brings to happiness and also we follow the Rayvah who says that there's need to lean nowadays since it doesn't express freedom by leaning. Magen Avraham 472:5 cites the Bach. Pri Megadim E"A 472:5 writes that he shouldn't lean on a fancy couch but should still lean. Mishna Brurah 472:13 agrees.
- Shulchan Aruch O.C. 472:7 writes that if one didn’t lean while eating or drinking one must eat or drink again. This is also the opinion of Rav Ovadyah in Sh”t Chazon Ovadyah 13. Rosh Pesachim 10:20 writes that if you didn't lean for the matza or four cups you should repeat those while leaning, even though repeating the third and fourth cups may make it seem like we drink more than four. Rama 472:7 adds that since we don’t drink between the third and fourth cup nor do we drink after the fourth cup one shouldn’t drink again because it appears as though one is adding to the established number of cups. However, one should Lechatchila repeat the first two cups if one forgot to drink while leaning. Magen Avraham comments that since the minhag is not to drink between the first and second cup (and so, if one does want to drink again one would make a new bracha and it’d appear like on is adding a new cup to the established seder) thus, one should also not repeat the first cup. Nonetheless, to avoid that issue the Magen Avraham suggests that one should have in mind before drinking the first cup that if he forgets to lean he’ll drink again. Mishna Brurah (Beiur Halacha s.v. VeNirah Li), Eliyah Rabbah, and Shulchan Aruch HaRav rule like the Magen Avraham. However, Bach and Avi Ezri argue on the Rama that one does not need to repeat any of the four cups of wine. So quotes the Bear Heteiv in the name of the Chok Yacov and Agudah.
- Rama 472:7 says that Bedieved one can rely on the Ravyah 525 that nowadays it’s not a practice of royalty to lean and so one fulfills their mitzvah without leaning.
- Pri Megadim E"A 485:1 explains that some hold that one doesn't fulfill the mitzvah even Biblically if one ate without leaning similar to Tosfot Sukkah 3a and Brachot 11a. Rav Ovadia in Chazon Ovadia v. 1 p. 197 points out that according to Ritva Sukkah 28a, Rashbetz Brachot 11a, Meiri and Ran Pesachim 116b who disagree with Tosfot one certainly fulfills one's Biblical mitzvah.
- Even though the initial requirement of Matzah during Motzei Matzah is 2 Kezaytim, Mishna Brurah 472:22 rules that one only needs to repeat to eat one Kezayit of matzah.
- Mishna Brurah 472:22 says that if one forgot to lean during Afikoman one shouldn’t eat it again while leaning because one can’t eat after Afikoman. However this only applies if one already said Birkat HaMazon, otherwise one should repeat the Afikoman, so comments Rav Nevinsal in BeYitzchak Yikrah on Mishna Brurah. However, Mishna Brurah 477:4 says that one should eat Afikoman again unless it’s difficult for a person. Accordingly, Sh”t Chazon Ovadyah 45 rules that one should eat the Afikoman again unless one already said Birkat HaMazon or it’s difficult to repeat. http://www.dailyhalacha.com/Display.asp?ClipDate=3/21/2010
- Shulchan Aruch 472:6. Kesef Mishna (Hilchot Chametz UMatzah 7) explains that the Shamash is obligated even though he's involved in the preparations of the meal because an Eved Ivri is supposed to be treated like a master.