Trigger and data-acquisition challenges at the LHC

We review the main requirements placed on the Trigger and Data Acquisition (DAQ systems of the LHC experiments by their rich physics program and the LHC environment. A description of the architecture of the various systems, the motivation of each alternative and the conceptual design of each filtering stage will be discussed. We will then turn to a description of the major elements of the three distinct sub-systems, namely the Level-1 trigger, the DAQ with particular attention to the Event-Building and overall control and monitor, and finally the High-Level trigger system and the online farms.