  The performance of the ATLAS muon trigger system is evaluated with
proton-proton ($pp$) and heavy-ion (HI) collision data collected in Run 2
during 2015-2018 at the Large Hadron Collider. It is primarily evaluated using
events containing a pair of muons from the decay of $Z$ bosons to cover the
intermediate momentum range between 25 GeV and 100 GeV. Overall, the efficiency
of the single-muon triggers is about 68 % in the barrel region and 85% in the
endcap region. The $p_{\text{T}}$ range for efficiency determination is
extended by using muons from decays of $J/\psi$ mesons, $W$ bosons, and top
quarks. The performance in HI collision data is measured and shows good
agreement with the results obtained in $pp$ collisions. The muon trigger shows
uniform and stable performance in good agreement with the prediction of a
detailed simulation. Dedicated multi-muon triggers with kinematic selections
provide the backbone to beauty, quarkonia, and low-mass physics studies. The
design, evolution and performance of these triggers are discussed in detail.
