  This article discusses the techniques used to select online promising events
at high energy and high luminosity colliders. After a brief introduction,
explaining some general aspects of triggering, the more specific implementation
options for well established machines like the Tevatron and Large Hadron
Collider are presented. An outlook on what difficulties need to be met is given
when designing trigger systems at the Super Large Hadron Collider, or at the
International Linear Collider
