In the 1990s, the MLA sector – archives, libraries, museums – was defined as a comprehensive sector. One of the reasons for this was the belief that the digitizing of institutional collections and catalogues would provide a seamless access to the cultural heritage for the general public. No one would any longer need to know in which institution a certain object was kept; everything would be available on the Internet. The questions in this dissertation depart from the expectations in the late 1900s aimed at the available systems in the MLA sector for the management and storage of information and at knowledge maintained in such systems. My point of departure is the idea that access to collections in the MLA sector – real or digital – is made possible through the catalogues. It is in the catalogues that the sector orders data and preserves knowledge collected about the objects. My most important conclusions deal with the management and production of knowledge in the sector. The data systems generally used in the MLA sector lack the indulgence needed to accommodate inherent variability of information. Systems must be based on exact sorting, uniform terminology and classification without deviations in spelling or interpretation. Furthermore, international projects and systems presuppose distinct translations between national terminologies and international classification systems to work for information retrievals. This is not the case with the museum systems here investigated, in opposition to, primarily, the libraries. The museum systems are characterised as distinctive instead of following uniformity principles and national or international taxonomies. An important conclusion about knowledge processing and knowledge production in the MLA sector is that the value of compiling digitalised data from many museums is limited. It is not possible to realize the political goal other than at a superficial level. The conclusions in a comparison between the three professional groups are that in the beginning of the 21th century an extensive part of the antiquarians confirm a thought style, according to Ludwik Fleck, which has another direction than that of archivists and librarians. Throughout the 20th century or in the last decades of the 20th century the two latter groups professionalised their roles both through changes in education and through new professional requirements which led to positions and attitudes in their work other than what happened in the museum sector. Museums are mainly research institutions with members of staff who in many respects lack a focus on increased accessibility of collections. Challenges formulated for museums of cultural history by the cultural politics of the 1970s have not been answered with great flexibility. The investigation in this dissertation demonstrates that archives, libraries and museums in many respects continue to pursue activities according to a thought style which was formed during the first half of the 20th century. Their positions in society, their administrational practice and expectations from the general public constitute stabilising and preserving factors. The MLA sector is not as comprehensive as the policy makers want to believe.