Compared to other composers’ extant letter exchanges, Gustav Mahler’s correspondence with approximately 5000 letters is one of the most extensive letter collections of the 19th and 20th centuries. In addition to family mail, written exchanges with friends, artists, publishers, artistic directors and institutions, the correspondence also includes intellectual dialogues with other composers such as Richard Strauss or conductors such as Felix Weingartner. The resulting network of letter correspondents locates, on the one hand, Mahler’s influence in the historical context while simultaneously making his sociocultural position as well as the accompanying period-specific variance recognizable. On the other hand, the number and content of the letters characterize Mahler as a person in a syncrisis between the composer's external image and the perception of his contemporaries. Mahler's letters also partially document the genesis and his struggle for finalizing his works. Furthermore, the letters provide insights into the rehearsal work and performance of other composers' works. The project therefore aims to create a basis for an edition of letters by recording and indexing all of Gustav Mahler's letters that have so far appeared in print as well as the existing counter-letters. However, since the text-critical edition of the letters would far exceed the scope of the project, the focus of this undertaking lies in recording the meta-information of the letters while referencing the letter text.