The paper attempts to present a model of a circulation system indicating a spectrum of functions and benefits. Four types of circulation systems are discussed in an attempt to measure the capabilities of these systems against the model and each other. The model circulation system outlined is an on-line real time system in which the circulation file is created from the shelf list and the terminal inquiry system includes the capability to query and browse through the bibliographic system and the circulation subsystem together to determine the availability for circulation of specific documents, or documents in a given subject area, or by a certain author, etc. Most of the subsystems are updated on-line, and there is minimum delay to the user. The system is designed independent of the input medium. It may be an IBM 357 data collection system, a terminal keyboard, an OCR system, etc. The only requirement is the user does not have to be present to borrow an item. The model extends beyond the operational limits of most existing circulation systems and can be considered a reflection of the current state of the art.