LHC trigger & DAQ - An introductory overview

In this paper I will review the 4 large LHC Trigger and DAQ systems, focusing on the technological and engineering challenges and trying to bring out commonalities in both technology and approach, without omitting to hint at interesting variations. In the trigger part the emphasis will be on implementation and the specific LHC challenges, the physics will only be touched upon as much as is necessary to motivate design decisions. As the LHC experiments are preparing the first major round of upgrades, this is a very good moment to look at lessons learned from the operation so far, at successful approaches which will be continued and at new, promising ideas.