USER is a software estimating tool to assist software engineers in estimating the effort required to perform software enhancement and modifications in response to customer requests.  USER was developed by Abacus for a major computer and systems company which has the responsibility for maintenance of literally hundreds of programs used for Space Shuttle analysis.  The development of reliable, realistic, and consistent estimates for this type of programming effort has long been a problem for systems organizations.  In the final analysis, empirical evidence indicates that the best estimates are usually not derived by any "system" of estimating but rather they are the product of knowledgeable experts who are familiar with the software involved.  USER, being an expert system, follows the same types of strategy that such experts use in their approach to the estimating process.  Such an estimating system has several advantages over human estimators. It can arrive at a reasonable estimate more quickly; it can look at what was done in the past and determine if an estimate is reasonable; and it is not in danger of leaving and taking its knowledge and experience with it.  Further, USER will be able to learn from its past estimates to improve its accuracy in the future.