  The "Underlying Event" at CMS (under nominal and start-up conditions) is
studied by examining charged particle and momentum densities in the
"transverse" region in charged particle jet production. The predictions of
various QCD models with different multiple parton interaction schemes correctly
reproduce Tevatron data, however they fail to agree with each other when
extrapolated to the LHC energy. The possibility of discriminating among these
models is presented. Exploring QCD dynamics in proton-proton collisions at
center-of-mass energy of 14 TeV, and the importance of improving and tuning the
QCD Monte Carlo models at start-up are also analyzed.
