  We describe the design, construction and performance of the upgraded DO muon
system for Run II of the Fermilab Tevatron collider. Significant improvements
have been made to the major subsystems of the DO muon detector: trigger
scintillation counters, tracking detectors, and electronics. The Run II central
muon detector has a new scintillation counter system inside the iron toroid and
an improved scintillation counter system outside the iron toroid. In the
forward region, new scintillation counter and tracking systems have been
installed. Extensive shielding has been added in the forward region. A large
fraction of the muon system electronics is also new.
