Elazar Menachem Man Shach (אלעזר מנחם מן שך) (or Rav Leizer Shach, at times his name is written as Eliezer Schach in English publications) (January 22, 1898 - November 2, 2001), was a leading Eastern European-born and educated Haredi rabbi who settled and lived in modern Israel.
He was the rosh yeshiva ("dean") of the Ponevezh yeshiva in Bnei Brak, and founded the Degel HaTorah political party representing Lithuanian Ashkenazi Jews in the Israeli Knesset, many of whom considered him to be the Gadol HaDor ("supreme religious leader of the generation") and used the honorific Maran ("[our] master") when referring to him.
He was recognized as a Talmudic scholar par excellence by scholars such as Rabbi Yitzchak Zev Soloveitchik (the Brisker Rav) and Rabbi Isser Zalman Meltzer in their approbations to his works; he authored the Avi Ezri a commentary on the Mishneh Torah.

Life in Europe
Shortly before the start of World War II and the Holocaust, several yeshivas began considering evacuating their rabbis, students and families.Rabbi Kotler eventually left for America, travelling across Siberia and arriving in the United States during the war. In 1939,Rav Shach first went to Vilna, where he stayed with Rabbi Chaim Ozer Grodzinski. Later that year both Rav Shach's mother and eldest daughter fell ill and died. In early 1940 the Shach family decided to leave Lithuania. Rav Shach's maternal uncle, Rabbi Aron Levitan, had helped Rabbi Kotler get emigration visas, but Rav Shach instead decided to go to Palestine, where Meltzer was serving as Rosh Yeshiva at Etz Chaim Yeshiva in Jerusalem, (Rav Shach would later serve as Rosh Yeshiva there as well). His uncle helped Rav Shach and his family get emigration certificates and took them in after they arrived at his doorstep, destitute.
Several years after the re-establishment of the Ponevezh yeshiva in Bnei Brak, he was asked to be one of its deans. He remained in the position until his passing. At this Yeshiva, Rav Shach taught many thousands of students, many of whom eventually assumed prominent positions as Roshei Yeshiva and Rabbis.

Escaping to the British Mandate of Palestine
Shach was credited by many for helping revolutionize the concept of the "society of learners" in the post-war Haredi world. Under his leadership, the phenomenon of Haredi men studying in yeshivas and kollels full-time, something that had been comparatively rare in Europe before World War II, became the standard in many Haredi communities in Israel, with the financial backing of Haredi communities and subsidies to young families with many children from the Israeli government.

Political life
Shach was involved in a number of public disputes with Rabbi Menachem Mendel Schneerson the Rebbe of the Chabad-Lubavitch movement from the 1970s through Schneerson's death in 1994. Shach accused his followers of false Messianism. When once asked which religion was theologically closest to Judaism, Shach responded "Chabad"..
In addition to Shach's objections to some Chabad members venerating Schneerson as the Messiah (both before and after his death), the two also disagreed on various issues of Jewish law and philosophy, but particularly politics. Chabad strongly opposed peace talks with the Palestinians or to relinquishing any Israeli territory under any circumstance, while Shach alternately supported both left and right-wing parties in the Israeli elections. During the 1988 elections, Schneerson endorsed Agudat Israel over Shach's newly-formed Degel HaTorah party, and instructed Israeli Chabad to campaign for it. Shach's newspaper, Yated Ne'eman, ran several articles documenting various Chabad writings and statements which described Lubavitch as becoming a breakaway sect of Judaism focused around Schneerson as the Messiah.
There have been similar concerns regarding Chabad Messianism that have since been raised among Haredi and Modern Orthodox communities in Israel and the United States.

Family

"I remember how I was educated in my parents' home: when my yarmulke fell off my head, I was taught that you had to cry from distress. They were guided by a concern for the punctilious observance of mitzvos. Once I woke up after the zman Krias Shema according to the Magen Avraham and I burst out crying and continued to cry about it all day long."