The Catalogue of Carl Nielsen’s Works (CNW), published online by the Danish Centre for Music Editing, is the first ever thematic-bibliographic registration of all Nielsen’s compositions. It is based entirely on the so-called MEI Header (or metadata section) and was edited using the MerMEId tool. It includes all the information usually found in printed thematic catalogues such as instrumentation, date of composition, a short description of each work’s history, a list of movements including incipits, as well as lists of sources, performances, letters, diary entries and other documents. Source descriptions are kept at a minimum in the Nielsen catalogue since they are readily available in the complete edition of Carl Nielsen’s works, also accessible for download directly from the catalog. Other catalogs in preparation will contain more detailed descriptions of the sources. A printed Carl Nielsen catalog is in preparation, derived from the online version.