This thesis deals with the protection and preservation of old and valuable written heritage in libraries through the digitization process. At the beginning of the paper, the importance of protecting old and valuable written heritage is discussed, as well as concepts in the field of protection, and the most common damage to written materials, also mentioning legal frameworks and protection standards. Further, the traditional approach in the field of protection and preservation is briefly presented, followed by the discussion on protection of valuable written heritage by means of digitization. The digitization process and long-term digital preservation are also discussed. The example of the digitization project initiated by the Zagreb City Libraries in 2007 under the name Digitized Metropolitan’s Heritage is shown. The aim of the project was publishing digital copies, thereby preserving the heritage while increasing the visibility and accessibility of the fragile originals. The digitized collections of the Zagreb City Libraries, the Collection of old and valuable heritage - Rara, the collection Zagrabiensia and the Protected Collection of Children's Books are briefly described. To investigate the topic, employees of the City of Zagreb Libraries involved in the digitization process, the head of the digitization process and managers of collections of protected materials,were interviewed. This thesis emphasizes the importance of digitization process for the purpose of protection of valuable written heritage that the library holds and the importance of building digital collections through which the original written heritage continues to be used and promoted, but also to the challenges that librarians face in the effort of long-term protection and preservation of digitized material.