The topic of collaboration in school is very important and attracts the attention of many professionals. Although there are numerous studies on teacher-parent collaboration, there are just a few covering the topic of collaboration between professional associates. The school librarian is often a neglected member of school's professional team, which is why it was interesting to explore his collaboration with other professional associates. In the Republic of Croatia the role of the school librarian in education is very important because his work focuses mostly on the educational process. Therefore, many opportunities for collaboration will open up in the process. This paper covers theoretical knowledge about the school library, the school librarian and his role in the school as well as aspects and activities of collaboration. Furthermore, it presents the classification of collaboration aspects into six categories which make a successful collaboration. As a part of this paper a research has been conducted on collaboration of the school librarians in Croatia within their schools. The goal of the research was to examine the collaboration of the school librarians with professional associates in the Republic of Croatia, with special focus on the frequency of their collaboration, as well as on the ways and tasks they collaborate on and what their views on collaboration are. The results were obtained by giving the subjects of the research a questionnaire. The analysis of the results has shown that the school librarians rarely collaborate with other professional associates, but they are satisfied with the quality of their collaboration and have positive views on it. The areas of collaboration are very diverse. There is no correlation between independent variables (the size of a school, the years of service) and the frequency and views of the school librarians on collaboration.