Control and diagnostic tools for the event-filtering system (trigger) of the L3 experiment at the LEP (Large Electron Positron) collider have been implemented using an expert system. The structure and behavior of the trigger electronics has been modeled using the hybrid framework (allowing both frame-based design and rule-based reasoning) of the shell NEXPERT Object, which also provides a user friendly interface. An open architecture allows the integration in the system of conventional and widespread tools, like procedural languages and database management systems, and an easy interface to the whole online environment. The project goals, base model, development tools, and present implementation and future development of the trigger control are described. >