RASHI's family tree (the highlights indicate the tosafists)

The tosafot cover 38 tractates of the Talmud. Exactly where and when they were compiled, their types, and their historical and literary development are among the most difficult problems in the study of rabbinic literature. The tosafists, as the approximately 300 scholars who compiled this literature have come to be called, are generally considered to have originated in 12th-century northern France. Among them were Rashi's grandchildren, the two most famous of whom are R. Yaakov ben Meir Tam (Rabbeinu Tam, generally considered the chief architect of the tosafot, and the driving force behind them) and Shmuel Ben Meir (Rashbam, c.1085-1174). Other well-known Isaac b tosafists are Asher ha-Levi, Shimshon Ben Avraham of Sens, Yehuda Ben Yizhak of Paris, and Shmuel (Sir Morel) of Falaise.

The early tosafists both elaborated upon and continued the development of their teacher's line-by-line commentary on the Talmudic text (kunteres). Most tosafot begin by citing Rashi and pointing to the difficulties in his interpretation before presenting an alternate perspective. In this way they produced new halakhic deductions and conclusions, which in turn became themselves subjects for discussion, to be refuted or substantiated in the later tosafot. This tosafist method quickly became a dominant force that shaped the method of learning the Torah for centuries, first in Germany and France, and later moving to Spain as well.

Moses Nahmanides (Ramban) was undoubtedly the first to introduce the study of the tosafot into Spain, and his pupils and their pupils after them, Solomon b. Abraham Adret and Yom Tov b. Abraham Ishbili, established its study there. His contemporary, Asher b. Jehiel, who had come from Germany to Spain with his sons, was the second scholar to bring the study of the tosafot to Spain, thereby encouraging and advancing the process already flourishing there.

The early printers had already included the tosafot as the companion commentary to Rashi's commentary; wishing to enhance the value of their product, they accordingly printed the tosafot at the side of the page. To this day, a page in the Talmud is frequently referred to as (Ga-Pa-T): an acronym for the page's three components: the Talmud text itself (gemara), Rashi's commentary (perush) and the tosafot.