Haseiba: Reclining at the Seder
One of the requirements of the Seder is that of "haseiba," to recline on one's left side while performing many of the Seder rituals. This is because in the era of the Roman Empire, reclining, especially while eating, was a sign of royalty and freedom which are the themes of the Seder night. As the Talmud states, "even a poor man in Israel may not eat until he reclines" which conveys to us the importance for every single Jew to feel like royalty on the Seder night. This is also the reason that it is customary to arrange for someone else to fill and refill one's wine goblet for each of the four cups rather than to do so oneself.
It is also suggested that reclining while eating recalls the circuitous route by which God led the Jewish people in the desert towards Eretz Yisrael. This is alluded to by the word used to describe the journey, "vayasev," which is phonetically related to the word "haseiba." There is even a Midrash that teaches that the Jews were reclining upon beds as God took them out of Egypt.
Although reclining is only mandatory when eating the required portions of matza and when drinking the four cups of wine, there are those who recline at other points in the Seder, as well. There are also those who eat the entire festive meal while reclining. Most people do not recline while eating the marror. There is a difference of opinion whether one should recline or sit upright while reciting the haggada.
In order to properly discharge the haseiba requirement one must lean one's body weight upon something such as a chair or pillow, or even upon another person. It is not sufficient to merely lean towards the left. One should recline specifically at the Seder table rather than on a couch or recliner that is located at a distance from the table. This is true even if moving to such a couch would allow one to recline in a more enhanced manner. Indeed, one is required to prepare the seating arrangements at the Seder table early in the day on Erev Pesach in a manner that will allow everyone to properly recline at the Seder.
In ancient Roman times, almost the entire body was outstretched when people, especially the wealthy upper class, would eat. This is because they ate while reclining upon couches, especially at festive meals. At the Seder, however, it is only necessary to lean at a 45 degree angle. One should only recline after one has recited the blessing upon the wine or other food that one is about to eat. The reason to recline on one's left side is that most people eat with their right hand. The other reason is that it is believed that leaning on one's right side while eating poses a choking hazard. As such, even those who are left-handed should recline on their left side, even if doing so might cause some inconvenience while eating. Nevertheless, a left-handed person who reclined to the right has fulfilled his obligation.
As a general rule, women are not required to lean at any time during the Seder, though some authorities rule that a "distinguished woman" should lean. There are a number of interpretations as to which women are considered "distinguished" in our day. Some authorities rule that all women are considered "distinguished" today. Alternatively, it means the daughter or wife of a distinguished man. Other interpretations of "distinguished" include an unmarried woman, a righteous woman, and a woman who has servants at her disposal. Nevertheless, common custom in Ashkenazi communities is for women not to lean during the Seder while in most Sefardic communities they do.
There is an opinion that there is no longer a requirement to recline at the seder nowadays, since reclining while eating is no longer the manner in which royalty conducts itself. The halacha, however, is not in accordance with this view. In fact, one who ate the required servings of matza or drank the four cups of wine without reclining might be required to eat and drink those items again. It is interesting to note, however, that one of the reasons why Ashkenazic women don't recline at the Seder is in deference to the view that reclining at the Seder is not required at all nowadays. Due to the requirement for one to revere one's father and rabbi, a son in the presence of his father, or a student in the presence of his rabbi, should obtain permission before reclining at the Seder.
 Pesachim 99b, 108b; Rambam, Hilchot Chametz U’matza 7:8.
 Kaf Hachaim, OC 472:12.
 Rema, OC 473:1.
 Shemot 13:18.
 Shemot Rabba 20:19.
 Mishna Berura 472:22.
 Rema, OC 472:7.
 Mishna Berura 475:14.
 Mishna Berura 473:71; Siach Yitzchak 216.
 Mishna Berura 472:7; Or L'tzion 3:15, Rivevot Ephraim 2:129:32.
 Piskei Teshuvot 472:1.
 OC 472:2. See also Rashi, Berachot 46b s.v. "b'zman"; Rashi, Shabbat 43a s.v. "l'mizga".
 For more on this see: http://en.wikipedia.org/wiki/Triclinium.
 Halichot Shlomo, Pesach, 9:20 note 44.
 Mishna Berura 472:10.
 Mishna Berura 472:10.
 Terumat Hadeshen 1:136; OC 472:3; Mishna Berura 472:11; Kaf Hachaim, OC 472:23, 48; Hitorerut Teshuva 2:49.
 OC 472:3; Mishna Berura 472:10,11.
 Pesachim 108a; OC 472:4. See also Divrei Yatziv, OC 5.
 Beit Yosef, OC 472.
 Rivevot Ephraim 6:248:1.
 Rabbeinu Mano'ach, Hilchot Chametz U'matza 7:8. See also Haggada Shleima p. 70-72; Halichot Shlomo, Pesach 9:19; Moadim U'zmanim 3:257; Igrot Moshe, OC 5:20.
 Rema, OC 472:4.
 Kaf Hachaim, OC 472:28; Chazzon Ovadia, Pesach Vol. 2.
 Ra'avia 525, Tur, OC 472:2; Rema, OC 472:7.
 OC 472:7; 477:1; Mishna Berura 472:21,22; 477:4; Kaf Hachaim 472:42,45; 477:7. See also Pesachim 108a; Tzitz Eliezer 18:27; Minchat Yitzchak 9:46,47.
 OC 472:5; Be'er Heitev, OC 472:5; Teshuvot V'hanhagot 1:306.