When the Three Mile Island Unit 2 accident occurred, personnel were dispatched to the site from all over the world to render assistance. The large influx of people (on the order of several thousand persons) placed a tremendous burden on the plant operators’ radiation exposure management (REM) system, which was not designed, or staffed, to handle the volume of transactions that was being generated. Also, due in part to the increased volume, but to an even greater extent the unique characteristics of the accident situation from a radiological and logistics perspective, there were many new, previously unanticipated, health physics information management needs and requirements being generated on a continual basis. This situation precipitated literally hundreds of requests for REM system changes. Many of the requested changes were very extensive and complex in terms of overall logistics and information flow and would have been expensive to implement under the current design. It soon became apparent that the best approach would be to completely redesign the existing REM system to meet the special requirements imposed by the accident situation. The decision was made to design and develop a totally new REM computer system that employed “on-line” transaction processing concepts being used in other industries such as banking and retail. The major problems and decisions that influenced the design and development of GPU Nuclear Corporation’s current on-line computerized REM system (REM on-line) are discussed.