This article will focus on R. Jacob ben Meir, better known as Rabenu Tam, grandson of Rashi (France 1100–1171 of the Tosaphists) and his use of the Talmud Yerushalmi. The Yerushalmi was not in extensive use in the middle ages. This article will first describe the use of the Yerushalmi by the European sages and then will try to depict R. Tam’s knowledge of Yerushalmi and the purposes for which he quotes this Talmud. Although R. Tam had at least some of the Yerushalmi tractates in his library, his main allusion to the Yerushalmi is from secondary sources such as R. Hananel, R. Nisim Gaon and others. This article will claim that R. Tam’s interest in the Yerushalmi was not methodical and encompassing. He did not treat the Yerushalmi as an authoritative Halakhic source.