Rabbi Isaac ben Jacob Alfasi ha-Cohen (1013 - 1103) also known as the Alfasi or by his Hebrew acronym RIF (Rabbi Isaac al-Fasi), was a Talmudist and Posek (decisor in matters of halacha - Jewish law). He is best known for his work of halacha, the legal code Sefer Ha-Halachot, considered the first fundamental work in halachic literature. He was born near Fes in Morocco, and spent the majority of his career there, and is therefore known as "Alfasi" ("of Fes" in Arabic).

Alfasi was born in Algeria. He studied in Tunisia under Rabbeinu Nissim Ben Jacob and Rabbeinu Chananel Ben Chushiel the recognized rabbinical authorities of the age. Rabbeinu Chananel trained Alfasi to deduce and to clarify the Halacha from Talmudic sources, and Alfasi then conceived of the idea of compiling a comprehensive work that would present all of the practical conclusions of the Gemara in a clear, definitive manner. To achieve this goal, he worked for ten consecutive years in his father-in-law's attic.

In 1045, the "heterodox" sects in the city were severely persecuted, and the Jews, with the rest, suffered greatly. As a result, the city's Jewish residents fled. Alfasi moved to Fes with his wife and two children. Fes' Jewish community undertook to support him and his family so that he could work on his Sefer Ha-Halachot undisturbed. They also founded a yeshiva in his honor, and many students throughout Morocco came to study under his guidance. The most famous of his many students is Rabbi Yehuda Halevi, author of the Kuzari. He also taught Rabbi Yoseph ibn Migash (the Ri Migash), who was in turn a teacher of Rabbi Maimon, father and teacher of Maimonides (Rambam). Alfasi remained in Fes for 40 years, during which time he completed his Sefer Ha-Halachot. In 1088, aged seventy-five, he left Fes for Spain, eventually becoming head of the yeshiva in Lucena in 1089. In a sense, Alfasi brought the geonic period to a close. The last of the Babylonian geonim, Rav Hai Gaon, died when Alfasi was 25 years old.

Sefer ha-Halachot, also referred to as "the Hilchot of the Rif", extracts all the pertinent legal decisions from the three Talmudic orders Moed, Nashim and Nezikin as well as the tractates of Berachot and Chulin, 24 tractates in all. Alfasi transcribed the Talmud's halakhic conclusions verbatim, without the surrounding deliberations. He also excludes all Aggadic matter as well as discussion of the halakha practicable only in Land of Israel. Maimonides wrote that Alfasi's work "has superseded all the geonic codes... for it contains all the decisions and laws which we need in our day."

Sefer ha-Halachot plays a fundamental role in the development of Halakha. Firstly, "the Rif" succeeded in producing a digest, which became the object of close study, and led in its turn to the great Codes of Maimonides and of Rabbi Joseph Karo. Secondly, it served as one of the "Three Pillars of Halakha", as an authority underpinning both the Arbaah Turim and the Shulchan Aruch. Rabbi Nissim of Gerona (the RaN) compiled a detailed and explicit commentary on this work. In yeshivot "the Rif and the RaN" are regularly studied as part of the daily Talmudic schedule. This work was published prior to the times of Rashi and other commentaries, and resulted in a profound change in the study practices of the scholarly Jewish public in that it opened the world of the gemara to the public at large. It soon became known as the Talmud Katan ("Little Talmud").