The purpose of master's thesis was to examine the practice of subject cataloging in different Slovenian libraries. The aim of the research was to get a general overview of subject cataloging in order to find at least some source of disunity. We wanted to establish reasons for different results in subject description between different types of libraries and to suggest ways for achieving greater unity of subject cataloging. As the method for gathering data we used a questionnaire. In our study we have also included results of the seminar paper that students of library science wrote for the course Subject cataloging in the academic year 2013/2014. Their sample included 11 public, 16 primary school, 15 secondary school and 10 academic libraries. We added 8 academic and 8 special libraries. This data was collected from February to May 2016. We found out that most of the cataloguers almost always assign subject headings and classification numbers. In order to do it, they use methods such as skimming through the book's content and often also informing themselves about the content elsewhere. Most of the cataloguers in Slovenian libraries use the subject heading list called the Slovenian general list of subject headings (Slovene: Splošni slovenski geslovnik - SSG) and classification scheme Universal Decimal Classification. According to the cataloguers, library users often use subject headings and rarely or almost never use classification numbers when they're searching for material. The reasons for the lack of unity are mainly located in non-usage of guidelines, instructions and manuals for cataloging by subject headings and classification numbers or in usage of different kind of aids. What could also bring greater unity to subject cataloging is a unified list of subject headings and classification numbers that would be used in subject cataloging by Slovenian cataloguers.