Posek (Hebrew: פוסק [poˈseq], pl. Poskim, פוסקים[pronunciation?]) is the term in Jewish law for "decider"—a legal scholar who decides the Halakha in cases of law where previous authorities are inconclusive or in those situations where no halakhic precedent exists.
The decision of a posek is known as a psak din or psak halakha ("ruling of law"; pl. piskei din, piskei halakha) or simply a "psak". In Hebrew, פסק is the root implying to "stop" or "cease"—the posek brings the process of legal debate to finality. Piskei din are generally recorded in the responsa literature.
Formulating a ruling (psak din)
In formulating a ruling, a posek will base the psak din on a careful analysis of the relevant underlying legal principles, as well as a careful study of the application of these principles. A Posek must therefore be thoroughly versed in rabbinic literature, especially the Babylonian Talmud.
The process of analysis usually entails today:
- an initial study of the relevant Talmudic Sugyas with commentaries;
- tracing the development of all related material in the Rishonim (Medieval rabbinic authorities prior to the Shulkhan Aruch) through the Arba'ah Turim and Shulkhan Arukh;
- and finally, a close analysis of the works of the Acharonim (rabbinic authorities from about the 16th century onwards) discussing the halakha as recorded in the literature of the Rishonim (and earlier Acharonim).
The ruling itself is an attempt to apply the precedents and principles of the Tradition to the question being asked. One common goal of poskim in this regard is, as far as possible, to be consistent with the codified law, as well as with the maximal relevant legal precedents, generally being decisions recorded in the responsa literature.
The role of the Posek
- In the Haredi world, each community will regard one of its poskim as its Posek HaDor ("Posek of the present Generation").
- Hasidic Jews rely on the Rov in their community (sometimes but not always Rebbes also get the position as Rov) or leading posek recommended by their Rebbe. Yet there are some Jews that are Hasidic but are not part of a specific movement these hassidim will vary in who they follow sometimes following generic hassid-style poskim like Rav Shmuel Wosner.
- Modern Orthodox Jews may select a posek on a more individual rather than a communal basis, although customs vary.
The approach taken here will, generally, be as above. Thus poskim will not overrule a specific law, unless based on an earlier authority: a posek will generally extend a law to new situations, but will not change the Halakhah; see further under Orthodox Judaism.
Conservative Judaism and Masorti Judaism approach the idea of Posek, and Halakha in general, somewhat differently, and Poskim here apply a relatively lower weighting to precedent, and will thus frequently re-interpret (or even change) a law through a formal argument; see Conservative Halakha. Although there are some "poskim" in the Conservative movement - e.g. Rabbis Louis Ginzberg, David Golinkin, Joel Roth, and Elliot Dorff - the rulings of any one individual rabbi are considered less authoritative than a consensus ruling. Thus, the Conservative movement's Rabbinical Assembly maintains a Committee on Jewish Law and Standards, whose decisions are accepted as authoritative within the American Conservative movement. At the same time, every Conservative rabbi has the right as mara d'atra to interpret Jewish law for his own community, regardless of the responsa of the Law Committee.
The role of the posek is somewhat different in Progressive Judaism, as these movements stress individual autonomy for interpretation of biblical and oral law. Nevertheless, since these movements consider Jewish law for various decisions, responsa on halakha have been written by some Reform Rabbis, including Solomon Freehof and Walter Jacob. Full text collections of Reform responsa are available on the website of the Central Conference of American Rabbis. 
List of Orthodox poskim and major works
Poskim of past years
- Yehezkel Abramsky (1886–1976)
- Shlomo Zalman Auerbach (1910–1995, Minchat Shlomo)
- Yoseph Chaim of Bagdad (1832–1909, Ben Ish Chai, Rav Pealim)
- Meir Brandsdorfer (Kaneh Bosem) (1934–2009)
- Avraham Danzig (1748–1820, Chayei Adam)
- Mordechai Eliyahu (1929–2010)
- Chaim Pinchas Scheinberg
- Yosef Shalom Eliashiv (1910-2012)
- Yechiel Michel Epstein (1829–1907, Aruch ha-Shulchan)
- Moshe Feinstein (1895–1985, Iggerot Moshe)
- Shlomo Ganzfried (1804–1886, Kitzur Shulchan Aruch)
- Avraham Gombiner (c.1633–c.1683, Magen Avraham)
- Chaim Ozer Grodzinski (1863–1940, Achiezer)
- Yisrael Meir Kagan (1838–1933, Mishnah Berurah, Chafetz Chaim)
- Avraham Yeshayahu Karelitz (1878–1953, Chazon Ish)
- Sabbatai ha-Kohen (1621–1662, Shach)
- Chaim Kreiswirth (1918–2001)
- Yechezkel Landau (1713–1793, Noda bi-Yehudah)
- Shneur Zalman of Liadi (1745–1812, Shulchan Aruch ha-Rav)
- Avraham Chaim Naeh (1890–1954 Ketzos ha-Shulchan, Shiurei Mikveh, Shiurei Torah)
- Ephraim Oshry (1914–2003)
- Chanoch Dov Padwa (1908–2000, Cheishev Ho'ephod)
- Menachem Mendel Schneersohn (1789–1866, Tzemach Tzedek)
- David HaLevi Segal (1586–1667, Turei Zahav)
- Yoel Sirkis (1561–1640, Bach)
- Moses Sofer (1762–1839, Chasam Sofer)
- Yaakov Chaim Sofer (1870–1939, Kaf ha-Chaim)
- Yonasan Steif, (1877–1958)
- Yoel Teitelbaum (1887–1979, Vayoel Moshe, Divrei Yoel)
- Vilna Gaon (1720–1797, Gra)
- Louis Ginzberg (1873–1953, The Responsa of Professor Louis Ginzberg)
- Isaac Klein (1905–1979, A Guide to Jewish Religious Practice)
- Jacob Agus (1911–1986, Dialogue and Tradition)
- Solomon Freehof (1892–1990, Reform Jewish Practice and its Rabbinic Background)
- Jacob Zallel Lauterbach (1873–1942)
- Eliezer Waldenberg (1917–2006, Tzitz Eliezer)
- Menashe Klein, (1924-2011, Ungvar Rebbe)
- Yechiel Yaakov Weinberg (1878–1966, Seridei Eish)
- Yitzchok Yaakov Weiss (1902–1989, Minchas Yitzchak)
- Eliezer Melamed
- Simcha Bunim Cohen, prolific author and pulpit rabbi in Lakewood, New Jersey
- Dovid Feinstein, rosh yeshiva at Mesivtha Tifereth Jerusalem
- Fishel Hershkowitz, Klausenburger dayan in Williamsburg, Brooklyn, New York
- Avigdor Nebenzahl
- Yehoshua Neuwirth (Shemiras Shabbos Kehelichasah)
- Hershel Schachter, rosh yeshiva at RIETS
- Gedalia Dov Schwartz, av beth din of Beth Din of America and the Chicago Rabbinical Council
- Osher Weiss (Minchas Osher)
- Moshe Landau (Rabbi of Bnei Brak)
- Mordechai Willig, rosh yeshiva at RIETS
- Shmuel Wosner (Shevet HaLevi)
- Ovadia Yosef (b. 1920, Yabbia Omer)
- Aharon Leib Shteinman
- Yitzchak Berkovits, rosh kollel The Jerusalem Kollel
- Nahum Rabinovitch, Rosh Yeshiva Yeshivat Birkat Moshe in Ma'ale Adumim, Israel
- Aaron Lichtenstein, Rosh Yeshiva, Yeshivat Har Etzion in Alon Shvut, Israel
- Zalman Nechemia Goldberg, Av Beit Din, Rosh haYeshiva of Machon Lev, editor-in-chief of the Encyclopedia Talmudit
- N. S. Hecht et al., An Introduction to the History and Sources of Jewish Law, Oxford University Press.
- Louis Jacobs, A Tree of Life: Diversity, Creativity, and Flexibility in Jewish Law, second edition, Littman Library of Jewish Civilization, 1999.
- Mendell Lewittes, Jewish Law: An Introduction, Jason Aronson Inc., 1994.
- Authority and Autonomy in Pesikat HaHalacha, archived from the 2004 original at nishmat.net
- An introduction to the system of Jewish Law, aish.com
- Jewish Law Research Guide, University of Miami Law Library
- Jewish Law: Examining Halacha, Jewish Issues and Secular Law (online journal)
- AskMoses.com, Live Answers