Today’s library acquisition, circulation, metadata management, cataloging and administration of all kind of resources are too laborious and cannot be performed manually. These tasks are run by or with the help of an IT-system. This master thesis aims to start an analytical discussion of the modern library's central tool, library IT-systems, a discussion much missing in Library and Infor-mation Science. The data on two library IT-systems Alma and Sierra was collected by partici-pant observations, interviews and document studies at Karolinska institute university library and Södertörns university college library.The data was analyzed through actor-network theory perspective. The study’s main questions are how the library IT-system actor-network is assembled and function, and what role does the technology play in library’s practices.The study shows that a complex actor-network emerges in order to automate and computerize library tasks. In this actor-network library IT-systems vendors, the library community, KIB’s and SHB’s practices and the technology are actors who actively impact and affect each other. They are associated with each other through negotiations, a process in ANT referred to as translation. The study also shows that as the actors are not completely in agreement with each other, the library IT-system actor-network has not yet transformed into a fully convergent actor-network which ANT compares to a black box. The study shows that nonhumans, the IT-systems, are not neutral tools. They mediate, organize and document library practices. But they are also a product of modern library knowledge.The study concludes that the fully functioning solution for libraries automated and computer-ized services is the joined production of human and nonhuman actor-network where IT technology play a mediating role.