The ATLAS transverse momentum trigger evolution at the LHC towards Run II

The missing transverse momentum triggers of the ATLAS experiment at the CERN Large Hadron Collider (LHC) are designed to select collision events with non-interacting particles passing through the detector. Such events provide an interesting probe for new physics interactions beyond the Standard Model, but also provide the basis for precise measurements of Standard Model parameters such as the Higgs couplings. The transverse momentum used in the trigger system is calculated from calorimeter-based global energy sums and supplemented with information from the muon detection system. The trigger operated successfully during the first running period of the LHC. With the start-up in 2015, the LHC is now operating at a higher centre-of-mass energy and increased luminosity, both making it challenging to improve on the Run I performance. A brief summary of the Run I performance studies will be presented, together with the Run II software and hardware-based improvements as well as some of the first results from the Run II data-taking period with $\sqrt{s} = 13$ TeV .