  The CMS muon system operates gas-based detectors. Upgrades of the detectors
and trigger components are needed to cope with increasingly challenging
conditions of the HL-LHC. New irradiation tests are performed to ensure that
the muon detectors will survive the harsher conditions and operate reliably.
The new CERN GIF++ (Gamma Irradiation Facility) allowed to perform aging tests.
We present results in terms of system performance under large backgrounds and
after accelerated aging tests. New detectors will be added to improve the
performance in the critical forward region: gas electron multiplier (GEM)
detectors will be installed in LS2 in the region 1.6 $<$ $|\eta|$ $<$ 2.4,
aiming at suppressing background triggers while maintaining high trigger
efficiency. Further enhancements are foreseen with a second GEM station and
with two stations of new generation RPCs, having low resistivity electrodes.
These detectors will combine tracking and triggering capabilities and can stand
particle rates up to few kHz/cm$^2$. In addition, to take advantage of the
pixel tracking coverage extension, a new GEM detector covering up to $|\eta|$ =
2.8. is foreseen behind the new forward calorimeter.
