The development and deployment experience of IMPACT, an expert system shell dedicated to the tasks of real-time network alarm correlation, is discussed. IMPACT was developed at GTE Laboratories and since 1991 has been used at GTE to build and field various telecommunications network (both wireline and cellular) event correlation applications. IMPACT incorporates an advanced real-time event correlation model which is based on the principles of model-based reasoning. The IMPACT application development environment contains multiple structural and graphical editors to describe network element classes, network configuration models, correlations, and correlation rules. IMPACT is a heavily graphics-oriented system with components to visualize network connectivity, present network events, and display geographic maps. IMPACT runs on UNIX workstations, including IBM RS600, Sun Microsystems SPARC, and SGI Indy. On SGI Indy R44OOSC, the performance of IMPACT reaches 15-30 events parsed and correlated per second. IMPACT has been integrated with two GTE network alarm monitoring systems, SmartAlert and ISM2OOO. IMPACT has been developed using C, X Windows, CLIPS, and Tcl/Tk. This paper summarizes the challenges of alarm correlation application development in the environment of a large telecommunications company, and states the benefits of using dedicated expert system shells.