The subject of the review is the triggering in medium scale experiments at high and intermediate energy accelerators. General requirements to trigger systems are discussed, and the features of such systems in concrete experiments allowing for effective event selection of the process in study are presented as well. The event selection methods based on energy deposit, multiplicity, time of flight, ionization losses, tracking information and other available in real time data are considered. The examples of using the multilevel trigger are given. The necessity of permanent control for the event selection process is stressed because its imperfections may result in irreplaceable loss of data.

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