The purpose of the paper is to investigate the phenomenon of openness in relation to library development. The term openness is presented and related to library development from historical and theoretical perspectives. The paper elaborates on the differences over time on to how openness has been understood in a library setting. Historically, openness in form of the open shelves played a crucial role in developing the modern public library. The paper examines this openness-centred library policy as adopted by Danish public libraries in the beginning of the 20th century by applying the theories by Michel Foucault on discourse and power to the introduction of open shelves. Furthermore, the paper discusses current challenges facing the modern public library in coping with openness issues that follow from changes in society and advances in technology. These influences and developments are not least brought about by the boom of the internet and the advent of the post-modern globalised knowledge-based and network-organised society. Finally, the paper outlines how theoretical and strategic library development can benefit from academic considerations on the dialectics between openness and restrictedness and on the transformed meaning and significance of openness in the knowledge society. The paper concludes that openness is still of decisive importance to library development and policy and that it should be included in strategic considerations on library development.