This document describes and compares on-line library circulation systems that have been reported as operational or developing, based on a non-comprehensive survey of published articles and public and restricted technical documentation. A checklist of components and features is used to tabulate if and how each system accommodates four selected functions (charge, discharge, renewal, and reserve) and if and how it uses any of four specially defined files (user file, item file, absence file, and log or transaction file). The emphasis is to document the data that are used; how they are stored, accessed, and transferred; and what messages, records, and circulation information result. Narrative synopses of the systems are also given. Complete accuracy and fairness are not claimed for the descriptions of systems presented in this document. (Author)