R. Isaac Meir Rothenberg Alter (1789–1866), the founder of the Gur dynasty, whose father R. Israel was a disciple of R. Levi Isaac of Berdichev and rabbi of Gur. R. Isaac Meir grew up under the tutelage of R. Israel Hofstein, the maggid of Kozienice, who influenced Isaac considerably. At an early age he distinguished himself in Torah study, showing originality and intellectual acumen. He subsequently studied under R. Aryeh Leib Zinz, rabbi of Polotsk, and won reputation as a brilliant young scholar. After the death of the maggid of Kozienice, and a short period with the latter's son and successor R. Moses, R. Isaac Meir left him to become a disciple of R. Simhah Bunem of Przysucha, and after his death, of R. Menahem Mendel of Kotsk. He continued to give unreserved support to R. Menahem Mendel throughout the stormy controversy which divided Kotsk Hasidism and during the period when R. Menahem Mendel was in isolation, enabling Kotsk Hasidism to survive its acute internal crisis. After R. Mendel's death in 1859 R. Isaac Meir was acknowledged as their rabbi by the majority of the Kotsk Hasidim. R. Isaac Meir displayed a ready awareness of public needs and was well acquainted with Jewish problems in Poland. He fought uncompromisingly to preserve the traditional Jewish way of life and headed opposition to the regulations imposing changes in dress issued by the government and upheld in Jewish circles by the maskilim, refusing to make concessions even when imprisoned by the authorities. During the Polish uprising of 1830 he was suspected of sympathizing with the Polish loyalists. He changed his name from Rothenberg to Alter. In his private life he experienced considerable suffering, losing his 13 children during his lifetime.

Although R. Isaac Meir derived the principal part of his teaching from the Przysucha-Kotsk school of Hasidism, in practice it revealed radical divergences. Instead of withdrawing from contact with the masses he tried to win them over, and interested himself in day-to-day problems. He made himself available to all who sought him out, receiving them kindly. However, like the Kotsk school he placed Torah study at the center of spiritual life. As one of the most eminent scholars in Poland of his day he developed among his followers enthusiasm for Torah learning. He also followed the Kotsk method in emphasizing profundity of thought in the search after truth and the inner promptings of the heart, and in continuous striving after self-perfection.

The period of his leadership, which lasted only seven years, had a formative influence on the development of Hasidism in Poland. Gur Hasidism became a powerful element in Orthodox Polish Jewry, and retained a leading position until the Holocaust. The Gur (Yid. Ger) hasidic dynasty, one of the most celebrated of the dynasties, existed in Poland from 1859 to 1939; subsequently the center moved to Erez Israel, under the Gur rabbi residing in Jerusalem. Gur Hasidism is based primarily on the trend in Hasidism developed by R. Jacob Isaac of Przysucha (Peshiskha) and R. Menahem Mendel of Kotsk (Kock) but has taken an individual direction. It also derives ideologically from the philosophy of R. Judah Loew b. Bezalel of Prague (the Maharal).