The aim of this paper is to explore and to make visible how the information work of hospital librarians is enacted in key practices where services of the hospital library are employed to support evidence-based practice. The empirical material was produced at three hospital libraries in three different regions in Sweden between January and March 2020. A practice-oriented approach using the theoretical lens information work is employed to analyze nine semi-structured interviews with hospital librarians and hospital library managers, together with field notes from observations of interactions between hospital librarians and healthcare practitioners. The analysis investigates the conditions for information work performed by hospital librarians as they participate in three key practices: clinical practices, information seeking practices, and HTA-practices. The results of the analysis are related to four categories of invisible information work, and the nature of the information work done to counter different types of invisibilities within the key practices is discussed. The findings suggest that a substantial amount of the information work of hospital librarians is invisible to clinicians. At the same time, considerable efforts are made by hospital librarians to counter different types of invisibility, for example through building relationships with healthcare staff and to develop and make specialized competencies visible. In particular, the importance assigned to evidence-based practice in healthcare allows for the librarians to be regarded by clinicians as legitimate partners with clearly defined competencies in specific situations.