A number of recent studies have examined the performance of concurrency control algorithms for database management system. the results reported to date. father than being definitive have tended to be contradictory in this paper rather than presenting yet another algorithm performance study we critically investigate the assumptions ,made in the models used in past studies and their implications. we employ a complete model of a database environment to study the relative performance of three different approaches to the concurrency control problem under a variety of modeling assumptions we show how differences in the underlying assumptions explain the seemingly contradictory performance results we also examine how realistic the varous assumptions would be for real database systems.