The aim of the paper is to investigate the preservation of written heritage in monastery libraries from legislative, institutional and organizational perspectives, and establish the necessary organizational prerequisites for improvement. Setting off from the presupposition that the library collections of monastery libraries are of immense cultural value, and can therefore be considered cultural good and part of Croatian written heritage, the paper discussed the need for a systematic approach to its protection, both on the operative level, in libraries themselves, and on the strategic level, by the authorities and other relevant institutions in the Republic of Croatia. In addition to the analysis of the legal and institutional frameworks and library collections preservation projects, three case studies were conducted in Franciscan monasteries in Mostar, Požega and Zadar, including interviews with their managers and one subject from the Croatian Institute of Librarianship. The case study aimed to investigate the context of the preservation of library collections in monastery libraries and provide answers to the following questions: how is the preservation in monastery libraries defined; how does it differ from the preservation in other libraries, and how is the preservation of collections in these libraries organized on institutional, local, and national levels? The research sets off from several core presuppositions: monastery libraries have valuable collections of Croatian and European written heritage; the heritage collections in monastery libraries are not investigated, organized, protected or presented at an adequate level; the responsibility for its preservation is not clearly assigned, there is not enough staff trained for preservation; the improvements in preservation and availability of library collections in monastery libraries should result from a better organization and management of the heritage preservation system in monastery libraries. The conceptual approach to heritage preservation in monastery libraries used in this paper is based on the theoretical model of written heritage preservation management that comprises five aspects bringing together the theoretical, strategic, legal, financial, educational, operative, and cultural components. Monastery libraries are facing many problems in protection and preservation of collections. Most of their problems fall under the economic, legal, material and operative aspects of preservation. In addition to the most common limitations, such as financial constraints, inadequate premises for keeping and storing the collections, they often have staff shortages, especially of trained staff with particular knowledge and skills required to protect the collections of such libraries, which brings the educational aspect of preservation into the foreground. Even though the protection and preservation of library collections in monastery libraries is mostly the responsibility of monastery monks, their knowledge is mostly not expert enough and they cannot be the only ones made responsible for preservation. This issue needs to be considered from the strategic and theoretical aspects of written heritage preservation management. One of the major hurdles is the inadequacy of legislation and its lack of recognition of those collections as national heritage. One of the factors that needs to be taken into consideration are changes that occur in the cultural and social contexts, i.e. changes in relations between church orders and communities in which they are active. Based on the data collected and relying on the conceptual model of written heritage preservation management in the Republic of Croatia, the current state of monastery library collections preservation has been established and measures for a systematic and more efficient management of written heritage preservation in monastery libraries have been proposed.