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.
