Rabbi Shmuel Dovid Halberstam, also known as Rabbi Shmiel Dovid Halberstam or the Sanz-Klausenberger Rebbe of Borough Park, is the younger son of Rabbi Yekusiel Yehudah Halberstam, the previous Klausenberger Rebbe who survived the Holocaust and reestablished the Sanz Hasidic dynasty in exile in both Israel and the United States, from his second marriage to Chaya Nechama Ungar. Following his father's death on June 19, 1994, leadership was divided according to his will between the two sons: the elder son, Rabbi Zvi Elimelech Halberstam, became Rebbe in Kiryat Sanz, Netanya, Israel, while Rabbi Shmuel Dovid assumed leadership of the Klausenburger faction that remained in the United States, centered in Brooklyn, New York, specifically in the Borough Park neighborhood. As the Sanz-Klausenberger Rebbe, he leads the Sanz Hasidic community in the United States, overseeing an extensive network of educational and charitable institutions, including the Mifal HaShas international organization which promotes the study of the Talmud. He is known for his scholarly depth, adherence to the rigorous spiritual and communal standards of the Sanz tradition—particularly in communal prayer and intensive Torah study—and his focus on Torah scholarship, Hasidic piety, and community welfare. He guides a devoted following within the ultra-Orthodox Jewish community, maintaining institutions for religious education, welfare, and ritual observance, while his influence extends across the Borough Park neighborhood and into various Sanz enclaves worldwide, maintaining a distinct identity within the broader Sanz-Klausenberger movement and coordinating with his brother on matters of dynastic importance. His role involves adjudicating religious matters, delivering discourses (shiurim) and addresses at major communal events such as the Siyum HaRambam—where he notably collapsed during a speech in February 2026 but recovered and continued—and fostering the growth of the community amid modern challenges. No major public controversies or legal issues are documented in available records.