The ATLAS Trigger in 2017 and 2018: Developments and performance

The ATLAS Trigger system has been operating successfully during LHC Run-2, between 2015 and 2018. Its excellent performance has been vital for the ATLAS physics program, selecting interesting collision events for a wide variety of physics signatures with high efficiency. This presentation gives a review of the ATLAS trigger system in 2017 and 2018, covering briefly changes compared to 2016, operational aspects and encountered constraints. Focus is put on the new event-topology-based selections at the Level-1 trigger and on the trigger performance at high pile-up seen in 2017 and 2018