Overview of trigger systems

This article presents an overview of trigger systems in existing and planned modern experiments, the physics observables used for triggering, and the techniques which are deployed to process the detector information within the often tight real-time budget. Examples of presented trigger systems include large-scale on-detector processing and readout systems based on microelectronics, systolic trigger processors based on field programmable gate arrays (FPGAs), high-rate cluster systems and high-level triggers, which typically perform full event reconstruction online.