- It is forbidden from the Torah for a man to seclude himself with a woman to whom he is forbidden. It is Rabbinically forbidden for a man to seclude himself with any woman, even a single woman. It doesn't matter if the woman is Jewish or non-Jewish.
Multiple Men and Women
- It is permitted for many men and women to be secluded together. What constitutes many men and many women? Some poskim write that only if there's 3 men and 3 women , some say 2 men and 3 women, and some say 2 men and 2 women constitute many men and many women. The poskim are general strict to consider it 3 men and 3 women.
- According to Sephardim, one man may not be secluded with multiple woman and one woman may not be secluded with multiple men. 
- It is permitted for a man to seclude himself with his mother, daughter, granddaughter, or wife even when she in her state of Niddah. It is permissible for a man to seclude himself with his sister, however, it shouldn't be done frequently.  A brother and sister still living in their parents home may be left alone together for only a few days and not for an extended period of time. 
- If a woman's husband is in the city, it's permissible to seclude with her unless one has a close relationship with her such as if one grew up with her or is a relative. 
- When one’s wife is with him, there is no problem of being alone with other women as well. Most poskim assume that this leniency applies to one's mother, daughter, and sister as well. Therefore, if a man is in a room with his wife, mother, daughter or sister, and one other woman he does not violate the yichud prohibition. 
Open to the Public
- It is permitted for a man and woman to be secluded with one another in an area that is open to the public except at nighttime.
- Some say that as long as the door is unlocked even if it is closed that area is considered open to the public, however, some say that it is only considered open if the door is actually open. Practically, some say that one can be lenient only if people in that area enter without knocking and getting permission. Some say that in cases of need one can be lenient regarding any rabbinical form of yichud.
- Some poskim hold that it is forbidden to seclude oneself with a woman which one is comfortable around in an area open to the public, while others permit it. 
- If it is only possible to see into a window from the street if someone from the street were to stand on a chair then the window doesn't make it that the room is considered open to the public. However, if the window is low enough that people from the street can see into the room normally or by simply going on their tippy toes then it is considered open to the public.
- Some say that it is forbidden for a man and woman to seclude themselves together even if there's a camera or webcam that makes everything in the room visible to onlookers in another location.  If someone is monitoring the camera, ome poskim say that one can be lenient. 
- According to Sephardim, if on the road at that time there's on average a car every 3 minutes and it is possible to see inside the car then there's no yichud for a man and woman together in a car. If the road isn't heavily traveled it is forbidden unless there are three men and three women, such as on a bus. However, on a bus where there was three men and three women and then some of them leave, if a woman is left alone with a male driver the woman must exit the bus unless there is a danger to get off. 
- According to Ashkenazim, some say that it is not proper for a man and woman to travel alone in a car, while many others say that it is only forbidden to travel alone on small streets, where there only a few cars on the road.
- It is permissible for a man to enter an elevator with a woman because the time in travel is usually very short. Some say that it is permitted if the elevator has a travel time of less than 3 minutes. 
- Many authorities hold that one man may not be in the same house as a woman even if they are in different rooms. 
Doctor and Patient
- For a dentist/doctor, yichud may be permitted if one is caught up in his work to the point that he will not do anything inappropriate. Some do not like this heter alone for yichud.
- A second potential heter for yichud for a dentist/doctor is that he/she will receive professional repercussions if he/she would act inappropriately. This suffices as a preventative measure.
- A third way to avoid yichud is by ensuring that other individuals can see inside the room or by simply making sure others are present.
- If when a dentist sees a patient he/she closes the door, if others commonly walk in, it could be muttar. But if other people never walk in when the door is closed, it would be assur.
- The relationship between a dentist/patient can be transformed into a libo gas ba situation if they develop a relationship through the professional interactions. Then these heteirim would not apply.
- Article on Selected Halachos relating to Parshas Achrei Mos by Rabbi Doniel Neustadt
- Yichud in Contemporary Times by Rabbi Ezra Schwartz
- Minchat Ish by Rav Ayal Sharga, the son of Rav Baruch Sharga, a major Sepharadi Rosh Kollel in Eretz Yisrael. (Volume One, Volume Two), useful for Sephardim in the absence of extensive Teshuvot from Rav Ovadia
- Gan Naul, another useful Sefer for Sepharadim
- Shulchan Aruch EH 22:1-2, Aruch Hashulchan EH 22:1, Kitzur Shulchan Aruch 152:1, Yalkut Yosef Hilchot Ishut EH Siman 22:1-2.
- The Gemara Kiddushin 80b and Avoda Zara 36b learns from a Pasuk that Yichud with a relative who one is forbidden to live with (someone who would be in the category of Arayot) is prohibited. The Gemara (Avoda Zara 36b) explicitly calls Yichud with a married women to be a biblical prohibition. Thus, Tur EH 22:1 writes that Yichud with an Erveh is biblical. The Rambam (Issurei Biyah 22:2), however, argues that Yichud is only Divrei Kabbalah. Bet Yosef (EH 22:1) explains that the Rambam holds that the gemara's expression that Yichud is biblical only meant that it is hinted to in the Torah.
- The Perisha EH 22 posits that there's a biblical prohibition to have yichud with someone who is forbidden on a Biblical level (lav).
- The Gemara Avoda Zara 36b says that the Bet Din of Dovid HaMelech instituted the prohibition of Yichud with a single woman and Bet Shamai and Bet Hillel extended Yichud to a non-Jewish woman as well.
- Shulchan Aruch EH 22:6
- Birkei Yosef EH 22:3, Pitchei Teshuva 22:5, Chelkat Mechokek EH 22:6 quotes this idea from the Maggid Mishna but questions it.
- Bet Shmuel 22:8, Chaye Adam 126:3
- Bach 22
- Igrot Moshe EH 65:15
- Yalkut Yosef Hilchot Ishut EH Siman 22:10. According to Yam Shel Shlomo Kiddushin Siman 20 and Sh"t Maharsham 3:152, one man with two women is a biblical prohibition. See however, Sh"t HaRashba 1:587 and Sh"t Chavot Yair 73
- Yalkut Yosef Hilchot Ishut EH Siman 22:1, Gan Naul pages 9-10.
- Gan Naul pages 10-12
- Yalkut Yosef Hilchot Ishut EH Siman 22:7
- Shulchan Aruch Even Haezer 22:5. Beit Shmuel 22:8 says that the reason for this leniency is that one’s wife is assumed to watch him to avoid sinning
- The Pitchei Teshuva EH 22:2 does not extend this Heter to one’s daughter (meaning a man, his daughter and a third person would violate Yichud), while Iggerot Moshe Even Haezer 2:15 extends it to a daughter and in Iggerot Moshe 4:65:8 extends it to a mother, and sister as well. The Nitei Gavriel Yichud 40:1 follows Rav Moshe (and in footnote 1 quotes many others who do as well).
- Kiddushin 81a, Shulchan Aruch EH 22:9
- Beer Heitev 22:9, Kitzur Shulchan Aruch 152:5
- Rashba (responsa 1:1251)
- Pitchei Teshuva 22:8 quoting the Beit Meir and Rabbi Akiva Eiger (responsa 100)
- Nitai Gavriel (Yichud 33:1), Dvar Halacha (responsa 26)
- Rabbi Uri Orlian in a shiur on yutorah.org (min 70-3)
- The Chelkat Mechokek 22:13 writes that a person who is comfortable around a certain woman is forbidden to seclude himself with her in an area open to the public. The Taz 22:9, however, disagrees and permits. Chida (Birkei Yosef 22:6 and Birkei Yosef 245:6) proves from Shulchan Aruch and others that it is permitted but he isn't willing to be lenient. Kitzur Shulchan Aruch 152:5 rules like the Chelkat Mechokek. Nitai Gavriel (Yichud 32:7) writes that in cases of need such as with a doctor one may be lenient unless the person is comfortable around the woman in an inappropriate sense. Gan Naul 8:25 writes seems to be lenient. He quotes the Shevet Halevi 5:23:7, Tzitz Eliezer 6:40:12:4, and Divrei Chachamim 5:11 as permitting, while the Nodeh Beyehuda EH 2:18, Aruch Hashulchan 22:6, and Igrot Moshe EH 4:60 are strict.
- Gan Naul 8:19-20 based on Tzitz Eliezer 6:40:11:8
- Kuntres Internet BeHalacha (p. 33) discusses whether having a webcam is similar to having a door open to the public domain and in the conclusion quotes Rav Elyashiv saying that it is not a valid leniency.
- yichud with video-camera. They also write that it is possible that it is also permissible if it is recording, even if nobody is watching. see also Rabbi Eli Mansour DailyHalacha. Rabbi Aryeh Lebowitz in a Ten Minute Halacha explained that it seems that a security camera that can be monitored at any time is enough to prevent yichud.
- Yalkut Yosef Hilchot Ishut EH Siman 22:9, Gan Naul 8:22. Gan Naul cites Rav Meir Mazuz in Or Torah 18:108:37 as agreeing.
- Rav Moshe in Sh”t Igrot Moshe EH 4:65:3 writes that initially it is not proper for a man and woman to travel alone because of a concern that they may stop in a deserted area and do something inappropriate. However, in a case of pressing need such as if a woman in the community needs to go to the same place he’s traveling and if he refuses he’ll be considered selfish, one may rely on those who hold that there’s no concern that they will stop in a deserted area.
- However, Rav Shlomo Zalman in Minchat Shlomo 1:91(21) writes that sometimes there's room to be lenient to rely on the fact that the car has windows which are visible to the public and that there's no concern that they may stop in a deserted place. (See there for more details). Additionally, Rav Vosner in Shevet HaLevi 5:202(1) writes that regarding the nighttime concern, there's room to be lenient since the person driving can't fall asleep, however, he adds that since sometimes intercity highways or even local streets become deserted at night one should be strict.
- Lastly, Rabbi Jachter quotes Rav Mordechai Eliyahu (cited in Techumin 10:311), Rav Aharon Lichtenstein, and Rav Mordechai Willig as ruling that it is permitted for a man to travel alone in a car with a woman unless they are driving in a very remote area or at a time when there are very few cars on the road. Nitei Gavriel (Yichud 44:1) rules that one roads where cars continue to pass by and the roads are well lit, there's no Yichud, however, on small streets where not very many cars travel it's forbidden. See further Rabbi Mansour on DailyHalacha.com.
- Rabbi Jachter quoting Teshuvot Igrot Moshe E.H. 4:65:16, Teshuvot Tzitz Eliezer 6:40:22, and Rav Shlomo Zalman Auerbach (Minchat Shlomo 1:91:22) say that it is permitted for a man to enter an elevator with a woman and it isn't yichud.
- Yalkut Yosef (Chinuch pg 389), Gan Naul 8:21. Rav Orlian mentions that potentially going up the glass elevators in CN towers in Toronto which are taller than 1100 ft and the elevators take 58 seconds is a bit of a question of yichud if a man and woman go in together.
- Sh"t Igrot Moshe 4:65:19. Rabbi Aryeh Lebowitz explains that even though Rav Moshe is strict there are some are opinions who are lenient but does not give a final ruling. See also Salmat Chaim 151, Chazon Ish 34:1-2, Shaar HaTziyun 239:27, Chachmat Adam 126:7, Divrei Malkiel 4:22, Tzitz Eliezer 6:40 (chap 7:10).
- Igros Moshe (E”H 4:65:1) says that a male OBGYN is so caught up in his work that he will not have any bad desires. He continues that even after he completed seeing the patient and other patients are waiting and he is not necessarily busy with the work at this moment, he doesn’t have time to linger around and must rush to the next patient, so there is no potential issur that he will do in that short time span. If it is the end of the day and no more patients are coming, he can rely on the secretary or someone else in the building to mitigate the issur yichud.
- Rav Shlomo Zalman Auerbach quoted in Nishmat Avraham 3:9 quotes the Gemara Kiddushin 80b which says that yichud applies even to a man and woman who are burying a baby in a cemetery, showing that there is an issur yichud even when one is busy with something where theoretically there should be no yetzer hara.
- Yaskil Avdi, Tzitz Eliezer, Nishmat Avraham, as quoted by Rabbi Mordechai Torczyner. Tzitz Eliezer says that this heter may potentially only apply when both individuals involved are busy with the work. In a dentist/patient relationship, only the dentist is preoccupied, so the heter may not apply.
- Igros Moshe E”H 4:65:2 says that if a window is low enough for people outside to see, there is no yichud. But if the window is very high to the point that the people outside must climb up a ladder to see inside, which is very unnatural, that would be an issur yichud. When it comes to a dentist/patient, if they are visible to the public through a window, that can potentially avoid yichud.
- Rav Moshe Shternbuch says that if one goes to a dentist/doctor of the opposite gender, one should bring his/her spouse or another relative.
- Rav Shlomo Zalman Auerbach
- Rav Shlomo Zalman Auerbach, quoted in Nishmat Avraham 3:94-95