Posek (Hebrew: פוסק‎ [poˈseq], pl. Poskim, פוסקים) 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.

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:

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.

Matisyahu Chaim Salomon is a rabbi and public speaker. He serves as the mashgiach ruchani of the Beth Medrash Govoha Yeshiva in Lakewood, New Jersey, United States. He is a lecturer on topics relating to Jewish religious growth and communal issues in the yeshiva world.

Salomon was born in Gateshead, England. He was educated in London. During his yeshiva and kolel years, he studied for 16 years with Rabbi Chaim Kaufman, who went on to found the Gateshead Yeshiva L'Zeirim. Rabbi Salomon also studied under Rabbi Elyah Lopian, the former rosh yeshiva of Etz Chaim Yeshiva for less than a year. Nevertheless Reb Elya is considered to be his main rebbe (mentor).

Salomon became mashgiach ruchani of Gateshead Yeshiva, a position he held for more than 30 years. He was mashgiach in Gateshead Yeshiva initially under Rabbi Moshe Schwab and then as the senior mashgiach, before moving to Lakewood in the spring of 1998. Rabbi Salomon was succeeded at Gateshead by Rabbi Mordechai Yosef Karnowsky.

A number of books have been published based on his public shmuesin (Mussar discourses) and shiurim (lectures) under the title Matnas Chaim on varying topics such as the Yomim Noraim (High Holidays), Moadim (holidays), Sha'arei T'shuva L'Rabeinu Yonah, Mesilas Yesharim, Shabbos & Rosh Chodesh, and Kinyanim. The book With Hearts Full of Love is based on a series of talks by Rabbi Salomon, edited by Rabbi Yaakov Yosef Reinman. The Jewish Observer published an interview with Rabbi Salomon on the Jewish approach to marriage.

Rabbi Yisroel Belsky is a prominent rabbi in the United States and a world recognized posek, who has served with the Orthodox Union since 1987. He resides in Brooklyn, New York. He is one of the lead roshei yeshivas in Yeshiva Torah Vodaas. He also serves as the rabbi of Camp Agudah, since the passing of the camp's former rabbi in 1967. Rav Belskys first kashrus position was with the Kof-K. He is the son of Rabbi Berel and Chana Tzirel Belsky. His maternal grandfather is Binyomin Wilhelm, the founder of Yeshiva Torah Vodaas.

Rabbi Belsky received his semicha from Yeshiva Torah Vodaath, in Brooklyn, NY in 1962, and from Rabbi Moshe Feinstein in 1965. He also studied in Beth Medrash Elyon for a number of years, having been one of its most prominent students. Rav Belsky considers himself a Talmid of Rabbi Yaakov Kamenetsky.

According to the New York Times, he is a "prominent and influential Orthodox rabbi in Brooklyn", and he is one of "the chief experts for the Orthodox Union, which certifies more than 600,000 products as kosher."