Summary: Citations play an important role in scientific discourse, in the practice of information retrieval, and in bibliometrics. Recently, there have been a growing number of initiatives which make citations freely available as open data. The article describes the current status of these initiatives and shows that a critical mass of data could be made available in the near future. New opportunities could arise from that, especially for libraries. The DFG funded project Linked Open Citation Database (LOC-DB) is presented as a practical way for libraries to participate.