The main purpose of this Masters thesis was to examine how the Internet and other electronic sources have affected public librarians use of printed reference sources. During the last couple of years the use of the electronic sources - in particular the Internet has become more established in the Swedish public libraries. Our aim was to examine if and how this has affected the use of the printed reference sources. This has been examined on the basis of the following factors: the gender and age of the librarians and how long they have served in the library. We also examined if the size of the library affects the use of the printed and the electronic reference sources and also which factors that are most important when librarians acquire reference sources. The last thing we examined was which reference sources the librarians used most frequently. The survey is based on a questionnaire that was sent to the 290 main libraries. One librarian in each library was asked to answer the questions. The answering frequency was 64,1 %. We also interviewed three librarians. The result shows that the printed reference sources are no longer used as much as they were before. Most often, librarians choose an electronic source first, but they often complement it with a printed source. Many librarians say that they dont prefer one format over the other; instead it depends on the question and the questioner. None of the factors that we examined seems to matter when the librarians choose format. Neither does the size of the library seem to affect the librarians use of printed and electronic reference sources. Possibly theres a tendency that the use has been affected more in larger libraries. The factors that librarians consider most important when they acquire new reference sources both printed and electronic are in the following order: content, cost, source and format.