The manuscript tradition of the Zaydi branch of Shiʿism, which since the 9th century has been preserved primarily in Yemen, is nowadays dispersed over countless libraries in Yemen and the Middle East, Turkey, Europe, and the United States, of which only a fraction has been digitized and is available for open access. Its treasures came to the attention of scholars outside Yemen at a relatively late stage. Whereas the bulk of Arabic manuscripts nowadays housed in the libraries of Europe were acquired between the 17th and 19th centuries in centrally located cities and regions such as the Ottoman capital Istanbul, Syria and Palestine, and Egypt—all strongholds of Sunnism—the collections of Zaydi/Yemeni manuscripts were established only at the end of the 19th and first decades of the 20th century. Among the European explorers and merchants who collected manuscripts in South Arabia and later sold them to libraries in Europe was Eduard Glaser, who visited Yemen on four occasions between 1882 and 1894. After Glaser sold the manuscripts purchased during his first and second journey to the Königliche Bibliothek zu Berlin in 1884 and 1887, Wilhelm Ahlwardt made them the last acquisition to be included in his Catalogue of Arabic Manuscripts, published between 1887 and 1899. The third Glaser collection was purchased in 1889 by the British Museum in London—with the exception of the Lane collection that was purchased in 1891 and 1893, it was the last acquisition to be included in Charles Rieu's Supplement to the Catalogue of Arabic Manuscripts published in 1894. The fourth Glaser collection was sold in 1894 to the Kaiserlich-Königliche Hofbibliothek in Vienna, constituting the most important acquisition of Arabic manuscripts by the library at the time—unlike the Berlin and London Glaser collections, the Vienna Glaser manuscripts were never described in a published catalogue. An even larger collection of Zaydi/Yemeni manuscripts was brought together by the Italian merchant Giuseppe Caprotti during his sojourn in South Arabia from 1885 to 1919. Portions of the Caprotti collection now belong to the Bavarian State Library in Munich and the Biblioteca Apostolica Vaticana, while the majority of the collection is owned by the Biblioteca Ambrosiana in Milan. European libraries and increasingly US libraries have continuously purchased manuscripts of Yemeni provenance during the 20th and 21st centuries.