National libraries hold large, valuable repositories of often unique materials. How can we ensure that the knowledge is accessible without endangering the materials that hold the knowledge? Digital preservation is meant to solve this problem. With more and more material being accessible digitally, users are expected to do more and more of the finding on their own, instead of being guided by librarians. What kind of knowledge do the librarians have, that cannot be mediated through a system? How is the knowledge of experienced librarians passed on to new librarians? What is the difference between physical and digital materials? I have interviewed senior and junior librarians of Det Kongelige Bibliotek in Copenhagen, Nasjonalbiblioteket in Oslo, the National Library in Helsinki, and Kungliga Biblioteket in Stockholm to learn about their journey to knowledge of their collections.