The article analyzes the principles of designing and forming digital collections of electronic libraries. Basic principles such as integration, strategic planning, quality assurance of digital content and its scientific and methodological support are considered. The need to create an expert council and working groups to determine the profile of electronic libraries and form the content of repositories is substantiated. The need to create an integrated electronic catalog of the cultural heritage of libraries, archives and museums to provide readers with information about the availability of archival documents and museum objects in library funds is emphasized. The main principles of its creation and functioning are given: use of a single data presentation format, use of a single method of description and indexing, use of single authoritative files to ensure control of access points; providing a single search interface. It was concluded that based on the results of theoretical research and practical activities of libraries in the field of digital collection formation, it is advisable to prepare a scientific and practical guide "Formation of digital collections", as well as to develop a program of lectures "Formation and use of digital collections.