In recent years interactive computer technology has significantly expanded the possibilities for users to control interfaces to group systems through individual or group modifications. Adapting the interface to individual work styles potentially enhances the ability of information workers to exploit more fully their substantive expertise and to enhance their job performance. However, user modifiability options have not been widely implemented, in part because of fears about impacts on organization.wide system integrity and manageability. This project conducts a controlled field experiment to study what happens to organizations, work groups, and individuals when users are allowed to modify system interfaces. About eight work groups of 15.20 people in two or more organizations are studied over eight months as they use electronic mail. Half the groups have a range of modification options; the others operate with unmodifiable systems. Effects at individual and group levels are studied using a combination of system.logged indicators, behavioral methods, and self reports. Analysis focuses on short and long.range performance effects, structural and communication effects, and the tradeoffs observed among outcomes. The study also characterizes the processes of implementation of modifiability options, and the diffusion of user.generated interface variations. The result sought is a more systematic picture of how user modifiability affects the implementation and use of information technology, as well as of the consequences for individual, group and organizational performance.