In 1936 Lieftinck successfully defended his doctoral dissertation on the Middle Dutch Tauler manuscripts.
[5]: 5 [3]: 20 Lieftinck participated in the founding of the prestigious cipl, Comité International de Paléographie Latine,[2]: 69 and played an important role in the design and implementation of the Catalog of Dated and Datable Manuscripts, a centrally important resource for codicologists.
[1] A member of knaw, Koninklijke Nederlandse Akademie van Wetenschappen (Royal Netherlands Academy of Arts and Sciences) from 1961, and a foreign member of kvab, Koninklijke Vlaamse Academie van België voor Wetenschappen en Kunsten (Royal Flemish Academy of Belgium for Science and the Arts) from 1964,[2]: 69 Lieftinck became his generation's principal authority in all matters relating to the Dutch and Latin manuscripts of the Netherlands.
[1] He published dozens of articles and books in Dutch, French, and English on topics in manuscript studies and palaeography.
[2]: 69 The first four volumes of the series Litterae Textuales (1: Varia Codicologica; 2: Texts and Manuscripts; 3: Neerlandica Manuscripta; 4: Miniatures, Scripts, Collections) were published in his honour.