From the website:
"The Yale Library Judaica holdings have grown slowly but steadily since the University's founding in 1701. Following the receipt of two major gifts in 1915, the Yale Library established a separate Judaica collection which is recognized as one of the major collections of Judaica in the country. The focus of the 95,000 volume collection, which includes manuscripts and rare books, is biblical, classical, medieval, and modern periods of Jewish literature and history, and supports the research needs of the faculty and students of the University's Judaic Studies Program and those of the broader academic community.
The social, religious, and cultural lives of the Jewish people are reflected in the Library's collections. Religious law, Sephardic studies, rabbinics, Jewish philosophy and modern thought, talmudica, and Hebrew, Yiddish, and Ladino languages and literatures are all represented in the collection."
Two collections are especially important for Yiddish Studies scholars:
The Yale University Library Catalog can be consulted here.