Abstract: An inventory monitoring system which is operative at a point of storage automatically to sense the presence and number of materials or product items stored in a storage volume generating a logical inventory map of the storage volume so that time-related mappings can be compared to detect item receipts and withdrawals. Preferred embodiments employ a novel electronic shelf unit equipped with a sensing grid or array of sensors that detect items supported on the shelf optically, or by pressure transducers or the like. Inventory information can be collected from multiple shelves and relayed to remote vendors or local computer systems for billing and reordering using infrared transceivers for communication between the shelves. A novel shelf supervisory unit has a modem to communicate remotely via the phone lines and an onboard processor to manage communications. The system has particular application to consignment marketing of surgical and medical supplies in medical suites.