  A measurement of the underlying event (UE) activity in proton-proton
collisions at a center-of-mass energy of 7 TeV is performed using Drell--Yan
events in a data sample corresponding to an integrated luminosity of 2.2
inverse femtobarns, collected by the CMS experiment at the LHC. The activity
measured in the muonic final state (q q-bar to opposite-sign muons) is
corrected to the particle level and compared with the predictions of various
Monte Carlo generators and hadronization models. The dependence of the UE
activity on the dimuon invariant mass is well described by PYTHIA and HERWIG++
tunes derived from the leading jet/track approach, illustrating the
universality of the UE activity. The UE activity is observed to be independent
of the dimuon invariant mass in the region above 40 GeV, while a slow increase
is observed with increasing transverse momentum of the dimuon system. The
dependence of the UE activity on the transverse momentum of the dimuon system
is accurately described by MADGRAPH, which simulates multiple hard emissions.
