R. Hayyim ben Alexander Suesskind Tschernowitz (Rav Za'ir, 1871–1949) was a talmudic scholar and Hebrew author. He was born in Sebesh (district of Vitebsk), Russia, studied in Lithuania and obtained semikhah from R. Isaac Elchanan Spektor of Kovno in 1896. Moving to Odessa the following year, he founded his own yeshivah, eventually transforming it into a rabbinical seminary (1907) which attracted many students from the Jewish intelligentsia in Russia, including Ḥayyim Naḥman Bialik and Joseph Klausner . R. Tchernowitz's ambition was to combine traditional study with modern research in order to rejuvenate Jewish learning. His pseudonym Rav Ẓa'ir (young rabbi) reflects his aims. Tchernowitz received a Ph.D. from the University of Wuerzburg in 1914. Settling in the United States in 1923, he taught Talmud at the Jewish Institute of Religion in New York.

Tchernowitz's writings may be classified under two headings: scholarly and publicistic. His first scholarly article appeared in Ha-Shilo'aḥ 3 (1898), entitled, "Ha-Sanegoryah be-Vattei Dinin shel Yisrael." He subsequently published studies on the codes of literature preceding R. Joseph Caro, "Le-Toledot ha-Shulḥan Arukh ve-Hitpashetuto" (Ha-Shilo'aḥ, 4 (1898); 5 (1899); 6 (1899)). In a more popular vein he wrote a series of general articles on the Talmud, "Ha-Talmud" (Ha-Shilo'aḥ, 7 (1901); 8 (1901–2); 10 (1902)), His first books were methodological studies aimed at modernizing the teaching of Talmud: Shi'urim be-Talmud (2 vols., 1903), on Bava Kamma, and Kiẓẓur ha-Talmud (vol. 1, 1919; vol. 2, 1922). Tchernowitz's primary interest was to produce a full historical account of the development of the halakhah. Although he did not discount the works of his predecessors, I.H. Weiss and I. Halevy , he thought that they neglected the long era preceding the late Second Temple period and that they overlooked sociological, ideological, and political factors. His concern was to present the halakhah not in its final crystallization but in its development beginning in pre-Mosaic times. His Toledot ha-Halakhah (4 vols., 1935–50) covers the period up to the destruction of the Second Temple, and Toledot ha-Posekim (3 vols., 1946–47) deals with the post-talmudic, geonic, and medieval periods. These works are widely used by students of the history of Jewish law. As a publicist, Tchernowitz showed deep interest in the Zionist movement and in contemporary Jewish problems. He published articles and essays, many of them controversial and polemic, in scores of Hebrew and Yiddish periodicals which appeared later in book form: Be-Sha'arei Ẓiyyon (1937). Ḥevlei Ge'ullah (1949) is a collection of his essays on the struggle for Jewish political independence. In 1940 Tchernowitz founded the Hebrew monthly Bitzaron in New York, which he edited until his death. Toward the close of his life he published a series of vivid autobiographical articles in Bitzaron, post-humously issued under the title Pirkei Ḥayyim (1954). His Masekḥet Zikhronot (1945) is a collection of essays on Mendele Mokher Seforim, Aḥad Ha-Am, Bialik, and other well-known personalities with whom he associated during the early stages of his career.