Public libraries are recognized as active participants in the cultural, economic, and social life of the community, and as such, they strive to encourage social responsibility in every individual. Nowadays, individual responsibility is closely related to the concept of sustainable development and concern for environmental protection and ecology. By incorporating these themes into library business, green libraries have emerged. The concept of green libraries appears in two different meanings and can refer to the library building itself or to activities related to sustainable development that the library conducts. In the Republic of Croatia, green libraries appear only in the latter meaning, so this paper deals exclusively with activities aimed at environmental protection and ecology conducted in public libraries. The first part of the paper contains definitions of key concepts and an overview of documents and guidelines that have influenced the recognition of libraries as promoters of these themes, as well as a review of the most prominent green programs. The second part of the paper is dedicated to research aimed at answering questions about the extent and ways in which public libraries in the Republic of Croatia conduct activities focused on environmental protection and ecology. The goal of the research is to provide an overview of aforementioned activities carried out from 2011 to the present. Central public libraries were selected as the sample for this research, and the method used is content analysis of the libraries' annual reports and their official websites. The results show the number of activities by libraries and years they were held, a comparison by the types of activities held, and an overview of the user groups they are intended for. The research conducted in this paper can serve as an incentive or idea for public libraries to improve activities aimed at environmental protection and ecology.