  A recent measurement by the D0 Collaboration, of the charge asymmetry of
muons in $W\rightarrow\mu\nu$ events is discussed. Using 4.9 fb$^{-1}$ of data
collected by the D0 detector, the muon charge asymmetry, $A(\eta)$, where
$\eta$ is the muon pseudorapidity, is measured inclusively and in two bins of
muon transverse momentum, \ptmu. After being corrected for background
contamination and detector effects (resolution and acceptance), the data are
compared to NLO QCD predictions with CTEQ 6.6 PDFs. For inclusive \ptmu,
reasonable agreement is observed. However, the prediction fails to describe the
data in bins of \ptmu. In most $\eta$ bins, the data are more precise than the
predictions, and will provide tight constraints on future PDFs.
