The Willem Witsen digital collection encompasses the artist’s entire correspondence. After the death of his second wife, Maria Schorr, more than 1200 letters addressed to him were found in the Amsterdam Oosterpark apartment. Among them were letters from Willem Kloos, Albert Verwey, Jan Veth, Marius Bauer, Frans Erens, Alphons Diepenbrock, Theodoor Nieuwenhuis, Jan Hofker, Frank van der Goes, George Breitner, Jacobus van Looy, Isaac Israels, Hein Boeken, Theodor Nieuwenhuis and many others. Since 1946, these letters have been in the safe keeping of the Koninklijke Bibliotheek, while the Amsterdam Universiteitsbibliotheek administers the correspondence (377 letters) between Witsen and his fiancée Elizabeth (Besty) van Vloten, whom he later married. About 800 further letters, mostly written by Witsen himself, are kept in other public institutions and by number of private persons. All in all, the letters constitute a corpus of more than 2000 documents. This digital collection has been supplemented with a good deal of visual material, such as the photographs Witsen himself took of his friends and relatives (ULB - Leiden University Library) and recent photographs of the work by Witsen and his contemporaries (RKD - Netherlands Institute for Art History). Together with the digital reproductions of personal documents, such as diaries and cash books (transcribing will render these accessible at a later date), the digital collection includes almost four thousand images.

The digital collection contains an extensive and coherent body of interesting source material which can be viewed and consulted by a vast group of users: it reflects not only the man Witsen, but also an artist’s life, a movement, a network and a period.

The following institutions, in possession of the original documents, contributed to this unique collaborative effort, which resulted in this extensive collection: National Library of the Netherlands, the Amsterdam University Library , the Netherlands Institute for Art History, the Management Committee of the Witsen Fund – Witsen House, the Print Collection of the Rijksmuseum, the Frans Hals Museum, the International Institute of Social History, the Literary Museum, the Netherlands Music Institute, the The Hague Municipal Archives, the Amsterdam Municipal Archives, the Archives of the Rotterdam Art Society, Museum Meermanno Westreenianum, the Netherlands Theatre Institute, the Leiden University Library and several private people. 

The transcriptions have been realised by the Willem Witsen Foundation, the Huygens Institute and the Digital Library of Dutch Literature. The Willem Witsen Foundation was established in 2003; its objective is to transcribe Witsen’s written legacy for the purpose of publishing it on a website. In recent years, the more than 2000 letters written by and to Witsen have been meticulously transcribed. All of the image presentations of letters to and from Willem Witsen feature a special 'transcription' button, enabling you to read the digital text version of the letters.