Rabbi Moshe Mordechai Epstein (1866-1934) was Rosh Yeshiva of Yeshiva Knesseth Yisrael in Slabodka, Lithuania and is recognized as having been one of the leading Talmudists of the twentieth century.

Rabbi Epstein was born in the town of Bakst, in the Vilna district of Lithuania, on the 20th of Adar, 5626 (1866), to Rabbi Tzvi Chaim and Baila Chana Epstein. His father, who served as the rabbi of Bakst, had been affectionately referred to during his days in the Volozhin yeshiva as "the Black Genius". Little Moshe Mordechai's genius was detected from a very early age. The child prodigy began studying in the Volozhin yeshiva at the age of 16, under the guidance of the legendary Torah giant Rabbi Chaim Soloveitchik. There, he met his brother-in-law-to-be, Rabbi Isser Zalman Meltzer, and, in 1889, married Menucha Frank, the eldest "Frank sister".